Subpage-emissions-process

Overview of Transportation Conformity Determination

The goal of a conformity analysis is to evaluate the impact of proposed transportation activities on a region’s air quality. The analysis involves developing a “budget” of the emissions likely to occur as a result of vehicle activity within a state, region, or locally. This budget must account for a wide range of factors that may affect the total emissions generated within a specified geographical area, and within a specified time period. Examples of factors that impact these emissions, and which must therefore be accounted for in a conformity analysis include:

1. Variability in traffic activity caused by short-term (day of the week) or long-term (regional population growth) trends.

2. Changes in the characteristics of vehicles using the road network. For example, age, engine size, and fuel types all impact the emission characteristics of a vehicle fleet within a region.

The results of a conformity analysis are submitted to the TCEQ and US EPA for their review before a long-range regional transportation plan or Transportation Improvement Program (TIP) is approved. The analysis must demonstrate that the emissions resulting from the plan or TIP meet the requirements of (i.e., “conform to”) air quality regulations. Therefore, while it is important to account for as many factors that affect emissions as possible, conformity analyses must also be objective and simple enough to be used for policymaking.

The clickable diagram illustrates a number of the steps involved in performing a conformity analysis. This process can be thought of as being conceptually simple, but complex in detail. The task involves four fundamental activities:

1. The development of emission rates for criteria pollutants: The pollutants of primary interest are: CO, Lead, Ozone, NO_X, PM₁₀, PM_2.5, and SO₂. Emission rates are calculated for different vehicle types, vehicle speeds, hours of the day, etc. These pollutants are emitted at different rates according to the type of activity associated with the vehicle (e.g., a running vs idling engine) and the source of emissions (e.g., exhaust emissions vs crankcase emissions). The process of estimating emission rates is illustrated in the panel of the clickable diagram.

2. Calculating traffic activity metrics: These metrics are driven by the nature of the roadway, and the traffic that uses the roadway. Four basic activity metrics are calculated:

   1. Vehicle Miles Travelled (VMT) is a measure of the total number of miles driven on a network link in a given time period. So if 10 cars were each to drive 10 miles on a road link in a given time period, the VMT would be 100.

   2. Starts is the number of occurrences of an engine starting on a road link in a given time period.

   3. Starts information is important because the emissions characteristics of a starting or cold engine are very different from those of a running engine.

   4. SHEI is a measure of vehicles that have extended idling engines. This category is used mostly for large diesel engines (trucks). Idling engines from commercial vehicles are a significant source of emissions in many areas.

   5. ONI is a measure of vehicles that have off-network idling activities. This category is used to estimate idle emissions that occur off the road network (i.e., on roadTypeID=1 in MOVES) for all soucetypes.

   6. SHP is a measure of the inactivity (parking activity) of a vehicle. Even when inactive, vehicles can emit significant pollutants through evaporative processes.

Because the amount of traffic on roads varies over both short and long periods, a conformity analysis must calculate these activity factors for a range of scenarios that include different days of the week, summer and non-summer periods (factoring in school traffic), current conditions, and years into the future. The panel of the clickable diagram provides more information on calculating traffic activity.

3. Calculating emissions for a road link: The total emissions generated by a link are calculated by multiplying the relevant traffic activity rates (Step 2) with the relevant emission rates (Step 1). The output of step 2 is actually a set of activity rates that represent different vehicle types and vehicle speeds that use the link (vehicle activity mix). To estimate total emissions, the activity characteristics of each of these vehicle activity mixes are multiplied by the emission characteristics of the matching vehicle mix that has been calculated in Step 1. An inventory of emissions is calculated for every pollutant, from every vehicle source, and for every category of the vehicle activity mix. This is repeated for every scenario under analysis.

4. Calculating total emissions for a region: The total emissions calculated for each link can be added together to calculate the total emissions generated over a larger geographic area (county, state).

5. Interpreting the results and demonstrating transportation conformity: Emissions estimated to occur under the proposed plan are compared to the established budgets for each pollutant/precursor. If estimated emissions exceed the budgets, plans and/or assumptions are adjusted, or other changes are considered. The emissions estimates are then recomputed. The iterative cycles repeat until conformity can be demonstrated (or it is concluded that conformity cannot be demonstrated).

This method of calculating emission inventories and total emissions provides a rational, quantified way of determining total on-road emissions and also provides insight into how different vehicle activities and vehicle types contribute to air pollution.

Emission Rates

On-road mobile vehicles generate emissions from a number of different sources. These include:

• Running and starting exhaust gases directly resulting from fuel combustion.

• Running and starting crankcase gases that are a by-product of fuel combustion.

• Evaporation of fuel and other hydrocarbons that occurs during regular vehicle use.

• Emissions from refueling vehicles.

Each emission source generates different types of pollutants, and in quantities that depend on a number of factors, such as:

• The type, age, or condition of the vehicle.

• The way in which a vehicle is driven (e.g., acceleration, deceleration, average speed).

• The number of times the engine is started and whether the engine is cold or warm when started.

• How much time the engine is inactive and idled off-network.

• How much time does the engine spend in extended idling.

• Fuel type (e.g., gasoline or diesel), and the formulation of fuel in an area (e.g., low sulfur fuels)

• The use of auxiliary powered devices (e.g., air conditioners, or truck refrigerator units).

• Environmental conditions (e.g. ambient temperature, wind speed, and road grade)

Emission estimation uses models, data, and expert knowledge to account for the most important processes that affect ‘real-world’ emissions in an objective and transparent way. This process requires that the complexity of the real world is abstracted or simplified enough to understand and validate the emission rate calculations. At the same time, the process must also ensure that enough detail is retained so that these models are representative of the ‘real world’. The level of detail appropriate for a conformity analysis is well established and has been developed through collaborative efforts between EPA, FHWA, state environmental agencies, state DOTs, and research agencies.

The first level of simplification for emission estimates is to simplify the analysis period into a number of scenarios relevant to understanding how emissions may change over the period of interest of the conformity analysis:

• Emissions are calculated for a region of interest (i.e., usually the county(s) in which is located the jurisdictional area of a non-conforming MPO)

• Emissions are calculated for appropriate yearly intervals that cover the temporal period over which the conformity should be demonstrated (20 years). This period begins with the current year (analysis year).

• For each analysis year, emissions are calculated for representative days from two seasons that objectively cover the range of environmental conditions (temperature and humidity) relevant to the study area (analysis season). This seasonal factor is also an important factor for traffic activity.

NOTE: In addition to year and season, the day of the week is also included in scenarios but mostly to represent important changes in traffic activity (see right portion of clickable diagram).

The second level of simplification defines the types of emissions, sources of emissions, and drivers of emission rates under the conditions defined by the aforementioned scenarios:

• Emission rates are calculated for up to 8 NAAQ pollutants or their precursors: CO, PM₁₀, Lead, Ozone, PM_2.5, NOx, and SO₂.

• Emission rates are estimated for a number of motor vehicle emission sources:

  • Exhaust and Crankcase emissions from running engines.

  • Exhaust and Crankcase emissions from starting engines.

  • Evaporative emissions from parked vehicles.

  • Emissions from extended idling of engines (heavy-duty trucks only).

  • Emissions from refueling.

• Vehicles are categorized into 24 different vehicle- /fuel-types.

• Emissions are calculated for different vehicle average speeds to the nearest 5 mph. Note that this factor only applies to emissions associated with running emission sources.

• Emission rates are calculated for typical drive cycles on each road class mapped to the MOVES Road Type Table. These drive cycles (starts, stops, accelerations, and decelerations) represent the differences in the way that vehicles are driven on different types of roads. Note that this factor only applies to emissions associated with running sources.

• Emissions are estimated for different temperature/humidity conditions representative of environmental conditions during alternative seasonal scenarios (e.g., summer and winter), and for different times of a typical day during these seasons. Emissions are calculated for every hour in each day (i.e., 1-24 hours).

• Emission rates are calculated specific to the formulation of fuels available within the region of interest and changes in this fuel availability throughout the analysis period.

• Emission rates are subject to projected changes in vehicle age distributions, vehicle types (e.g., average fuel consumption), emission control devices, and the projected impacts of emission reduction policies such as emission testing during vehicle inspections.

Texas emission rates are calculated using data and models provided by EPAs MOVES model. MOVES uses input data developed to represent the local conditions of the area being analyzed. However, since some areas do not have the resources to develop local data for all input factors, MOVES is furnished with default parameters and data for some of the parameters. A few of these input variables are:

• The number and types of vehicles registered in an area, and statewide.

• The formulation of fuels in a region.

• The environmental conditions of the area (average temperatures and humidity).

Once all these factors are accounted for, the analysis yields emission rates expressed in standardized units, and that can relatively easily be multiplied by traffic activity factors to provide estimates of total emissions. These emissions are specific to each pollutant of interest, vehicle type, road type (drive cycle), temperature, and humidity (represented by time of day and scenario season).

Emissions from Running Activity

The most visible source of on-road pollutants is those associated with the exhaust gasses of running vehicles. In a perfect combustion chamber, pure hydrocarbon fuels should burn with the oxygen in the air to produce energy, Carbon Dioxide (CO₂) and water (H₂O), which, along with the other constituents of air (predominately Nitrogen), would be emitted as exhaust gases. However, in reality, combustion is a complex physio-chemical process where the reactions that take place are driven by a number of factors, such as the formulation of the fuel, the operating conditions of the engine (e.g., altitude, temperature, humidity), the load placed on the engine (e.g., the weight and acceleration of the vehicle), and the age and condition of the vehicle.

In a conformity analysis, emissions from running vehicles are divided into two sources: exhaust emissions; and crankcase emissions. The former represents emissions that result from combustion in the engine’s cylinders. Crankcase emissions occur when hot exhaust gases and unburned fuel vapors escape from the cylinder into the surrounding crankcase and react with a much wider range of compounds. These emission sources are accounted for separately because each is associated with a different mix of pollutants and because different types of vehicles (depending on age, condition, and the presence of emission reduction devices) produce these pollutants at different rates.

Exhaust and crankcase emissions from a running vehicle represent the emissions that occur when an engine is “warm” and operating under optimum conditions (this situation contrasts with emissions that are generated when an engine is first started).

Emissions from running vehicles represent a significant proportion of total emissions from road vehicles. They are also most directly related to the movement of people and goods through the transportation network, i.e., to its actual function.

Exhaust and crankcase emissions of running vehicles are calculated in units of grams/mile for each pollutant of interest (criteria pollutants and precursors include CO, Lead, Ozone, PM₁₀, PM_2.5, NOx, and SO₂. The specific factors used to estimate accurate and representative emission rates for a vehicle are:

• Vehicle type- emissions are specific to each of the 22 categories of vehicles represented in the conformity analysis.

• Fuel formulation- representing the characteristics of the fuel available in a region.

• Speed- emission rates are calculated for speeds between 5 and 75 mph (in 5mph increments)

• Road type- the 18 categories of road type are used to represent different vehicle drive cycles.

• Temperature and humidity are defined by the seasonal scenario (i.e., summer or winter) and the time of day (hour of the day between 1 and 24).

• Any emission reduction devices written into the SIP, represented by changes in the emission rates associated with specific scenario years.

Off-network idling (ONI) emissions are another part of emissions from running vehicles. In EPA’s MOVES3 model, MOVES separately reports the emissions from the off-network idle hours in the movesOutput table as exhaust and crankcase running process (processID=1) for road type “off-network” (roadTypeID=1). total off-network idling emissions are obtained by multiplying the emission rates from running vehicles (expressed as grams/hour) by the Off-network idling hours (ONI) in units of hours.

As the clickable diagram shows, total emissions are obtained by multiplying the emission rates from running vehicles (expressed as grams/mile) by the vehicle miles traveled (VMT) in units of miles or the off-network idling hours in units of hours. For this process to be accurate, the factors relevant to VMT or ONI should be the same as those used to estimate the emission rates.

The emission rates may also change for different scenario years because of differences in the age structure of the vehicles in the vehicle fleet and policy changes, such as the introduction of emission testing during vehicle safety inspections or changes in fuel formulations.

Emissions from Starting Activity

Internal combustion engines are designed to operate most efficiently within a narrow range of operating conditions (warm engines) maintained by the engine’s heat management system. Catalytic converters also take time to reach optimum operating temperatures. When an engine is started, its ambient temperature is often below this operating range, leading to a period of higher emission rates.

They are explicitly accounted for in a conformity analysis because they have significantly different emission characteristics to other vehicle emission sources and because starting engines are an important component of real-world vehicle activity. As is the case for emission rates from running vehicles, emission rates from starting vehicles are further subdivided into pollutants from exhausts and crankcase sources.

As the clickable diagram shows, the emission rates (crankcase and exhaust) from starting vehicles (expressed as grams/start) are multiplied by the number of starts by the same vehicle type (under specific fuel formulation, temperature, and humidity conditions) to yield an estimate of total emissions. Note that the emission rates of these processes may change for different scenario years because of differences in the age structure of the vehicles in the vehicle fleet, anticipated policy changes such as the introduction of emission testing during vehicle safety inspections, or changes in fuel formulations.

• Vehicle type: emissions are specific to each of the 24 categories of vehicles represented in the conformity analysis.

• Fuel formulation: representing the characteristics of the fuels available in the region.

• Temperature and humidity: these factors are defined by the seasonal scenario (i.e. summer or winter) and the time of day (hour of the day between 1 and 24).

• Any emission reduction devices written into the SIP, represented by changes in the emission rates associated with specific scenario years.

The specific factors that are used to estimate accurate and representative emission rates for a vehicle are Exhaust and crankcase emissions from engine start activity, which are calculated in units of grams per vehicle start for each pollutant of interest.

Emissions from Extended Idling Activity

This emission category applies to combination long-haul trucks only. Drivers of these vehicles often idle their engines for extended periods for a number of reasons:

• During unloading or loading of goods.

• During rest periods when the idling engine is used to power auxiliary equipment such as refrigerator units, or cab devices that improve the comfort of the vehicle operator. This is especially important for long-haul truck drivers who are subject to a U.S. DOT’s rule that mandates that they rest 10 hours for every 14 hours of driving.

As in the case of emissions from running and starting engines, extended idle emissions are further split into exhaust and crankcase processes. They are explicitly accounted for in a conformity analysis for a number of reasons:

• They are associated with a warm engine, but because the engine is idling, it is assumed to be under minimum load.

• Extended Idling of trucks is an important component of traffic activity in a region.

Exhaust and crankcase emissions from idling engines are calculated in units of grams/hour for each pollutant of interest. The specific factors that are used to estimate accurate and representative emission rates for a vehicle are:

• Vehicle type- emission rates are only calculated for trucks.

• Fuel formulation- representing the characteristics of the fuels available in the region.

• Temperature and humidity are defined by the seasonal scenario (i.e., summer or winter) and the time of day (hour of the day between 1 and 24).

• Any emission reduction devices written into the SIP, represented by changes in the emission rates associated with specific scenario years.

Note that road type and speed, which are used as factors to estimate running emissions, are not relevant to calculating starting emissions.

As the clickable diagram shows, total emission rates from truck extended idling are estimated by multiplying the emission rates (crankcase and exhaust expressed as grams/hour) by an estimate of the total number of extended truck idling hours. Note that the emission rates of these processes may change for different scenario years because of differences in the age structure of the vehicles in the vehicle fleet, anticipated policy changes such as the introduction of emission testing during vehicle safety inspections, or changes in fuel formulations.

Emissions from Evaporative Processes

Evaporative emissions occur during all vehicle modes (i.e., while the vehicle is parked and running). These emissions are mostly hydrocarbon pollutants that escape into the air from fuel evaporation. Evaporation can occur from seals around fuel lines and other permeable sources. Developments of efficient emission control devices and fuel formulations have significantly reduced vehicle exhaust and crankcase emissions. In such cases, evaporative losses can account for a significant proportion of the total hydrocarbon pollution from vehicles. Evaporative losses are only considered for gasoline engines.

Evaporative emissions occur in several ways:

• Diurnal: Gasoline evaporation increases as the temperature rises during the day, heating the fuel tank and venting gasoline vapors.

• Running losses: The hot engine and exhaust system can vaporize gasoline when the car is running.

• Hot soak: The engine remains hot for a period of time after the car is turned off, and gasoline evaporation continues when the car is parked.

The principal pollutants for evaporative emissions are Volatile Organic Compounds (VOC) from fuel. Evaporative emissions are calculated in units of grams/mile and grams/source hours parked. The specific factors that are used to estimate accurate and representative emission rates for a vehicle are:

• Vehicle/Fuel type – emissions are specific to each of the 22 categories of vehicles represented in the conformity analysis.

• Fuel formulation – representing the characteristics of the fuels available in the region.

• Temperature and humidity – these factors are defined by the seasonal scenario (i.e. summer or winter) and the time of day (hour of the day between 1 and 24).

• Any emission reduction devices written into the SIP and represented by changes in the emission rates associated with specific scenario years.

Emissions from Refueling Activity

When a vehicle is refueled, emissions occur through two processes:

• For gasoline engines, vapors in the fuel tank can escape into the air passively but are also driven out of the fuel tank when new fuel is added. Note that neither process occurs when refueling diesel vehicles because it is not as volatile as gasoline.

• For both diesel and gasoline engines, emissions occur if fuel is spilled during the refueling process.

The principal pollutants for evaporative emissions are Volatile Organic Compounds (VOC) from fuel. Evaporative emissions are calculated in units of grams/mile and grams/source hour parked. The specific factors that are used to estimate accurate and representative emission rates for a vehicle are:

• Vehicle type- emissions are specific to each of the 22 categories of vehicles represented in the conformity analysis.

• Fuel formulation – representing the characteristics of the fuels available in the region.

• Temperature and humidity – these factors are defined by the seasonal scenario (i.e. summer or winter) and the time of day (hour of the day between 1 and 24).

• Any refueling emission reduction devices written into the SIP, represented by changes in the emission rates associated with specific scenario years.

Estimating Total Regional On-road Emissions

The methods of performing a conformity analysis involve understanding the total emissions that arise from different processes as a result of an on-road vehicle using the highway network. An important distinction that is often made between these distinct emission sources is between NETWORK (on-road) and NON-NETWORK (non-road) emissions. The former are emissions that occur while the vehicle is using the road link (running exhaust and crankcase, running evaporative), while the latter include emissions that occur in areas adjacent to the link (e.g., extended idling at warehouses, evaporative emissions from parked cars in driveways or places of work, refueling emissions at gas stations). Conceptually then, ON-NETWORK emissions can be thought of as specific to the spatial bounds of the road link they have been calculated for; while OFF-NETWORK emissions can be thought of as being emissions that are somewhat attributable to the vehicle activities on a particular link, but the source of the emissions might actually occur anywhere around the network.

ON-NETWORK emissions are calculated link-by-link. OFF-NETWORK emissions are calculated and accumulated on a non-link-based dummy road type for off-road activities of on-road vehicles.

These regional summaries can be disaggregated in a large number of different ways to report the quantities and trends of interest – for example:

• Total ON-NETWORK and OFF-NETWORK emissions for each pollutant of interest for each hour of every day of the week, season, and year scenario.

• Total emissions (both ON- and OFF-NETWORK) for each pollutant of interest for every hour, day of week, season, and year scenario.

• Total daily emissions per pollutant for each scenario day of week, season, and year.

This interpretation of the data provided by a conformity analysis is discussed in the ‘interpretation’ section of the clickable diagram.

Interpretation

Ultimately, a conformity analysis is performed in nonattainment and maintenance areas to demonstrate that the total on-road vehicular emission sources expected as a result of long-range (20-year) transportation plans do not exceed the established Motor Vehicle Emissions Budget (MVEB). These total vehicular emission estimates are representative of the region of interest and include calculations that account for changes in traffic activities associated with projected conditions in the analysis year, including any emission control strategies written into the SIP.

The MVEB is set by TCEQ and EPA in consultation with other agencies. It is set for each pollutant of interest, and for each year comprising the analysis period. The MVEB is set as a result of a complex, stakeholder-driven process that involves understanding the total emissions within a region (i.e., from on-road and off-road sources such as industry, and natural sources), but also the way in which total emissions (measured in grams) translate to concentrations of pollutants in the air via physical dispersion and photochemical processes (as determined through a photochemical modeling process that uses vehicle emissions estimates as an input.

Ultimately a conformity analysis involves comparing the projected emissions from vehicular sources with the maximum allowable budget of vehicle-specific emissions (MVEB):

• The MVEB details the upper limit of emissions per pollutant that should not be exceeded by the emissions estimated through the conformity analysis emission estimation methods.

• The conformity analysis estimates emissions likely to occur due to transportation over the same period. The conformity scenarios account for a realistic range of environmental and traffic conditions that drive vehicular emissions in a region.

Conformity is demonstrated if the total estimated emissions for a region for each day of the week, season, and year scenario are less than the value identified for the same year in the MVEB.

The difference between demonstrating conformity and reaching attainment is that a conformity demonstration only compares regional estimates of emissions to a regional emissions budget, while attainment must demonstrate through field measurements that air pollution concentrations at monitored locations actually meet established standards.

The goal of this overview is to provide a summary of the data and procedures that are important for estimating emissions for use in a conformity analysis rather than ALL the information that would be required to actually perform one. In reality, the data requirements, knowledge, models, and skills required to perform a full analysis are large and specialized and well beyond the scope of the simple computational summary presented herein.

Calculating Total Off-network Idle Exhaust and Crankcase Emissions From ONI Activity

The conformity analysis methods are designed so that the ONI calculations provide estimates that are associated with the same factors as those required to estimate running exhaust and crankcase emission rates, i.e.:

To calculate total emissions for running exhaust or crankcase processes, an ONI defined by the factors in the table above is multiplied by running exhaust and running crankcase emission rates calculated using the same factors that describe the VMT estimate. MOVES3 separately reports the emissions from the off-network idle hours in the movesOutput table as exhaust and crankcase running process (processID=1) for road type “off-network” (roadTypeID=1). Total off-network idling emissions are obtained by multiplying the emission rates from running vehicles (expressed as grams/hour) by the Off-network idling hours (ONI) in units of hours.

For example

1) An ONI estimate is calculated for a specific hour of the day and is associated with a number of vehicle activity factors summarized in the table below.

2) These same factors are used to estimate specific extended idle emission rates under these conditions for the pollutants of interest (highlighted data). This yields emission rates for each pollutant of interest:

3) These emission rates are multiplied by the ONI estimate to give total emissions from running vehicles for that vehicle type and hour of the day.

4) This process is repeated for all combinations of Vehicle Types and Time of Day to provide total running exhaust and crankcase emissions for 24 vehicle/fuel types and 24 hours of the day.

5) The total emissions for a link are then calculated by summing the emission estimates for all vehicle types for each hour of the day.

Calculating Total Exhaust and Crankcase Emissions From Running Activity

The conformity analysis methods are designed so that the VMT calculations provide estimates that are associated with the same factors as those required to estimate running exhaust and crankcase emission rates, i.e.:

To calculate total emissions for either running exhaust or crankcase processes, a VMT defined by the factors in the table above is multiplied by running exhaust and running crankcase emission rates calculated using the same factors that describe the VMT estimate.

For example:

1) A VMT estimate is calculated for a specific hour of the day and is associated with a number of Vehicle Activity factors summarized in the table below.

2) These same factors are used to estimate specific extended idle emission rates under these conditions for the pollutants of interest (highlighted data). This yields emission rates for each pollutant of interest:

3) These emission rates are multiplied by the VMT estimate to give total emissions from running vehicles for that vehicle type and hour of the day.

4) This process is repeated for all combinations of Vehicle Types and Time of Day to provide total running exhaust and crankcase emissions for 24 vehicle/fuel types and 24 hours of the day.

5) The total emissions for a link are then calculated by summing the emission estimates for all vehicle types for each hour of the day.

Calculating Total Exhaust and Crankcase Emissions From Starting Activity

The conformity analysis methods are designed so that the engine start emission estimates are associated with the same factors as those required to estimate exhaust and crankcase emission rates for running engines, i.e.:

To calculate total emissions for either starting exhaust or crankcase processes, engine starts, defined by the factors in the table above, are multiplied by starting exhaust and crankcase emission rates that are calculated using the same factors.

For example:

1) A START estimate is calculated for a specific hour of the day and is associated with a number of Vehicle Activity factors summarized in the table below.

2) These same factors are used to estimate specific extended idle emission rates under these conditions for the pollutants of interest (highlighted data). This yields emission rates for each pollutant of interest:

3) These emission rates are multiplied by the START estimate to give total emissions for the specific vehicle type and hour of the day (based on temperature and humidity).

4) This process is repeated for all combinations of Vehicle Type and Time of Day to provide total starting exhaust and crankcase emissions for 24 vehicle/fuel types and 24 hours of the day.

5) The total emissions for a link are then easily calculated by summing the emission estimates for all vehicle types for each hour of the day.

Calculating Total Extended Idle Emissions

The conformity analysis methods are designed so that the engine Extended Idle estimates are associated with the same factors as those required to estimate extended idle emission rates, i.e.,

For example:

1) A SHEI estimate is calculated for a specific hour of the day and is associated with a number of Vehicle Activity factors summarized in the table below.

2) These same factors are used to estimate specific extended idle emission rates under these conditions for the pollutants of interest (highlighted data), yielding emission rates for each pollutant of interest.

3) These emission rates are multiplied by the SHEI estimate to give the total emissions for that vehicle type and hour of the day.

4) Finally, the same calculations are performed for each vehicle type and for every hour of the day, yielding a matrix of total emissions by vehicle type and time of day (1-24 hrs) for that road link.

Calculating Total Evaporative Emissions

Evaporative emissions occur during all vehicle modes (i.e., while the vehicle is parked and running). Total evaporative emissions are calculated using SHP and VMT activity measures and can be divided into evaporative emissions from running vehicles and evaporative emissions from parked vehicles. Evaporative emissions are expressed in units of g / hour for parked vehicles and g/mile for running vehicles.

The conformity analysis methods are designed so that both SHP and SHO estimates are associated with the same factors as those required to estimate the emission rates associated with refueling activities, i.e.,

For example:

1) SHO and SHP estimates are calculated for a specific hour of the day, and associated with Vehicle Activity factors summarized in the table below.

2) These same factors are used to estimate refueling spillage and displacement (gasoline vehicle types only) emission rates under these conditions for the pollutants of interest (highlighted data). This yields emission rates for each pollutant of interest.

3) These emission rates are multiplied by the SHO and SHP estimates to give total running and parked evaporative emissions for that vehicle type and hour of the day.

4) Finally, the same calculations are performed for each vehicle type and for every hour of day yielding a matrix of total emissions by vehicle type and time of day (1-24hrs) for that road link.

Calculating Total Refueling Emissions

The conformity analysis methods are designed so that the refueling (RF) emissions estimates are associated with the same factors as those required to estimate the emission rates associated with refueling activities, i.e.,

For example:

1) A refueling emission estimate is calculated for a specific hour of the day and is associated with a number of Vehicle Activity factors summarized in the table below.

2) These same factors are used to estimate refueling spillage and displacement (gasoline vehicle types only) emission rates under these conditions for the pollutants of interest (highlighted data). This yields emission rates for each pollutant of interest.

3) These emission rates are multiplied by the REFUELING estimate to give total emissions for that vehicle type and hour of the day.

4) Finally, the same calculations are performed for each vehicle type and for every hour of day yielding a matrix of total emissions by vehicle type and time of day (1-24hrs) for that road link.

Off-network Idle

Off-network Idle is a traffic activity measure used to represent the total number of idle hours that occur off the road network (i.e., on roadTypeID=1) for all MOVES soucetypes.

For example, if five different vehicles each idled 3, 4, 6, 7, and 9 hours along a roadway, then the ONI would be equal to (3+4+6+7+9) = 29 vehicle hours.

However, different road links have a different composition of vehicle traffic, and each vehicle type has a different emissions profile. In reality, road link traffic comprises a mix of vehicle types (e.g., motorcycles, passenger cars, light-duty trucks, heavy-duty trucks, buses, etc.), makes, models, and ages. This mix of vehicle types is abstracted or simplified for a conformity analysis into 24 different vehicle-fuel types. These 24 different fuel and vehicle types are used to represent all the different types of vehicles that use the road link.

ONI is calculated for each road link in a network. The road link is classified into one of HPMS road types mapping to MOVES road types, and the average speed of all traffic on the road is estimated using the of the 24 vehicle-fuel types and for each hour of the day (1-24). This yields a data matrix of 24 vehicle types x 24 hours of the day (576 data points) (see Table 1)

ONI is calculated for every scenario in the conformity analysis (Year, Season, and Day of Week) and for each hour of the day.

ONI is calculated using High-Performance Traffic Monitoring Systems (HPMS—samples of traffic counts across the road network) and travel demand models that project future road network use.

Detailed link data are unavailable for some areas (mostly counties with small populations and vehicle activity). In such cases, methods have been developed to estimate link ONI using surrogate data.

Vehicle Miles Traveled

Vehicle Miles Travelled, or VMT is a traffic activity measure used to represent the total number of miles traveled along a road link in a given period (e.g., season/hour/day) by one or more classes of vehicles.

For example, if five different vehicles each traveled 3, 4, 6, 7, and 9 miles along a roadway in an hour, then the VMT would be equal to (3+4+6+7+9) = 29 vehicle miles traveled per hour.

However, different road links have a different composition of vehicle traffic, and each vehicle type has a different emissions profile. In reality, road link traffic comprises a mix of vehicle types (e.g., motorcycles, passenger cars, light-duty trucks, heavy-duty trucks, buses, etc.), makes, models, and ages. This mix of vehicle types is abstracted or simplified for a conformity analysis into 24 different vehicle-fuel types. These 24 different fuel and vehicle types are used to represent all the different types of vehicles that use the road link.

VMT is calculated for each road link in a network. The road link is classified into one of the HPMS link types mappings to MOVES road types, and the average speed of all traffic on the road is estimated using the 24 vehicle-fuel types for each hour of the day (1-24). This yields a data matrix of 24 vehicle types x 24 hours of the day (576 data points) (see Table 1)

VMTs are calculated for every scenario in the conformity analysis (Year, Season, and Day of Week) and each hour of the day.

VMTs are calculated using High-Performance Traffic Monitoring Systems (HPMS—samples of traffic counts across the road network) and travel demand models that project future road network use.

Detailed link data are unavailable for some areas (primarily counties with small populations and vehicle activity). In such cases, methods have been developed to estimate link VMT using surrogate data.

Vehicle Starts

MOVES default starts per vehicle are used in conformity analyses unless other location specific data is available. moves default start data does not vary by year or geographically but is affected by day type. Starts (expressed as number of vehicles starts per time period) are estimated for each vehicle type, and each hour of the day.

Source Hours of Extended Idling

Source Hours of Extended Idling (SHEI) is a measure of the total amount of time heavy-duty trucks spend with engines under extended idling (i.e., running but not powering a moving vehicle). In areas with high truck activity, SHEI is a significant traffic activity and can occur for a number of reasons:

• While trucks are waiting to be loaded or unloaded

• During mandatory driver rest periods. During rest periods, engines are often left idling to power auxiliary electrical equipment in the truck cab, or trailer cooling devices.

SHEI is calculated for trucks only and for each hour of the day. The truck specific VMT is an important factor in determining SHEI (i.e., if truck activity on a link is low, then truck idling is also assumed to be low). Empirical field data from commissioned studies also estimate extended truck idling hours per truck.

Source Hours Parked

Source Hours Parked is a measure of the total time that vehicles spend parked. It is an off-network traffic activity, meaning that the pollution does not directly arise from the road network itself; rather, its source is parking areas that are ancillary components of the road network. So, although it is technically not necessary to calculate off road emissions for each link, in practice it is often easier to do so, understanding that the emissions therein calculated are not necessarily directly applicable to that link. For example, if a region of interest contains 10,000 vehicles, and within a given 24-hour time period, 5,000 of these were parked, SHP would be:

SHP = 5,000*24hr

Because there are typically no data sources describing the actual number of parked vehicles in a region, it is estimated using the total number of vehicles in the network, VMT, and speed. Example 1 illustrates the equations used for a simple and trivial case that assumes 1 vehicle on a network. Example 2 uses the same principles but for a larger number of vehicles:

Example 1

Assuming 1 vehicle on the network, with a total VMT of 30 miles in 24hrs (meaning the vehicle has traveled 30 miles in a day), and an average speed of 30mph, off-network idling 1 hour:

• Total number of vehicle hours = 1 * 24 hours = 24

• Source Hours Operating (SHO) = VMT / speed = 30 / 30 = 1 hr.

• Off-network idling (ONI) = 1 hr.

• Source Hours Parked (SHP) = Total Vehicle Hours -SHO = 24 -1 -1 = 22 hrs.

This means that, on average, each vehicle on the network (in this case, there was only one) was parked for 23 hours of the day.

Example 2

Assuming 10,000 vehicles on the network, with a total VMT of 300,000 miles during a 24-hour time period (meaning that each vehicle has traveled an average of 30 miles in a day), and an average speed of 30mph, each vehicle off-network idle 1 hour:

• Total number of vehicle hours = 10,000 * 24 hours = 240,000

• Source Hours Operating (SHO) = VMT / speed = 300,000 / 30 = 10,000 hrs.

• Off-network idling (ONI) = 10,000 * 1 = 10,000 hrs.

• Source Hours Parked (SHP) = Total Vehicle Hours – SHO = 240,000 – 10,000 – 10,000 = 220,000 hrs.

• This means that, on average, each vehicle on the network was parked for:

  • 22,000 (SHP) / 10,000 (total vehicles) = 22 hours of the day.

In line with the other calculations, SHP is calculated for each hour of the day. This represents the fact that the temporal pattern of parked vehicles is negatively correlated with VMT activity (for example, most vehicles are parked at night when VMT on a network is low). Accounting for this temporal distribution is important because evaporative fuel loss emission rates (the emission source related to SHP) are strongly related to ambient temperatures.

SHP is calculated for each vehicle-fuel type and for each scenario in the conformity analysis (Year, Season, and Day of Week). Table 1 provides an overview of the data resulting from these SHP calculations.

Refueling

Refueling is an off-network activity because the pollution source is associated with gas stations and other sources ancillary to the road network.

The calculation of refueling activity is conceptually simple and is related to the total fuel consumption of traffic, and the average fuel tank volume of each vehicle type. Refueling is calculated for each vehicle type and for each hour of the day. MOVES provides default values for these refueling rates (that can be supplemented with local data), but the basic calculations are as follows:

Total fuel consumption arising from activity on and around the link is calculated as the sum of fuel consumption from VMT (moving traffic), Starts (the number of engines starts), and for trucks only SHI (Source Hour Extended Idling). Fuel consumption from each vehicle activity source is measured in total gallons:

• The fuel consumption from VMT is calculated by dividing the VMT (in miles) of a link by the average fuel economy (miles per gallon) of that vehicle type to yield the total gallons of fuel used by vehicles of a given type.

• Fuel consumption per vehicle start event, which is calculated using MOVES-derived default values derived from field measurements.

• For heavy-duty trucks only, the fuel consumption per SHI (source hour extended idling) is calculated using MOVES default values for fuel consumption during extended engine idling.

Given total fuel consumption, the number of refueling events (Refueling) attributable to the link is then calculated as:

• Refueling = Total fuel consumption / (average vehicle class fuel tank volume * F)

Where F is a factor that describes, on average, when fuel tanks are refilled (i.e. based on proportion of fuel left in the tank).. The equation yields the number of refueling events per hour of the day, as illustrated by the table below.

Traffic Activity

The emissions produced by on-road sources are a product of the number and types of vehicles using a road network, and the vehicles’ respective emission rates. In the ‘real world’, the vehicles that use the network are uniquely defined by factors such as their type (e.g., truck (several different classifications), passenger car, motorcycle etc.), their make and model (including engine size and specifications), age, and condition.

Vehicle emissions are also affected by the specific activities of the vehicles. Most obviously, emissions are associated with moving vehicles. Here, factors such as speed, and drive cycles (e.g., acceleration and deceleration) are important for accurately estimating emission rates.

However, significant emissions also occur during other vehicle related activities. These include the emissions from engine starts, extended idling of engines (especially heavy-duty trucks), refueling, and while vehicles are parked.

A major component of a conformity analysis is to estimate the total emissions generated by on-road traffic in a specified region. This is achieved by multiplying emission rates from each emission source by a corresponding measure of traffic activity. The traffic activities used in a conformity analysis are:

Ultimately, total emissions are driven by both the emission rates of different vehicles and the number and types of activities of different vehicle types using a road network. These traffic activity factors are calculated for each road link in a network for each hour of the day and in line with the scenarios that are used in the conformity analysis. The factors that make up a scenario are:

• Year: Representing long-term changes in vehicle engine technology, activity, or changes to the network.

• Season: This represents different traffic activities in the winter and summer seasons (which in turn determine school vs. non-school traffic).

• Day of Week: Representing changes in traffic activity that occur on different days of the week. The 4 days of the week used in an analysis are:

  • Monday, Tuesday, Wednesday, Thursday (Monday to Thursday weekday)

  • Monday, Tuesday, Wednesday, Thursday, Friday (Monday to Friday weekday)

  • Friday,

  • Saturday,

  • Sunday.

The emission rates from vehicles differ not only by emission process but also because of factors such as the vehicle type, speed, drive cycle, fuel formulation, temperature, and humidity. To ensure an accurate analysis, each traffic activity measure must be calculated along with a number of emission rate factors that are used to look up the emission rates specific to a vehicle type and its operating conditions. These factors are:

• Road type: each road link is assigned to one of 19 functional classes (the same as those used in the emission rate calculations), which represent vehicles’ drive cycles.

• Vehicle type: traffic activity measures are calculated for each of the 24 vehicle-fuel types that are also used for emission calculations.

• Average Speed: each road in the network is assigned a speed for each season, day, and time of day, based on projected traffic conditions. For emission rate calculations that use speed as an input (running exhaust and crankcase), all vehicles on the link are assumed to travel at the same speed during a given time period.

• Hour of day: traffic activity measures are calculated for each hour of the day. When combined with scenario season, this determines the temperature and humidity conditions needed to calculate accurate emission rates.

The clickable diagram provides more information on the methods used to estimate the different traffic activities.