3 Speed Estimation and Forecasting Examples

3.1 Background - Importance of Speed Estimates

Emission rates can vary widely with vehicle speed. Figure 1 shows the "speed correction factors" used in MOBILE6 for freeways for Tier 1 vehicles, which are used to scale emission rates from their base value (at 19.6 mph) to a value appropriate for a given speed. Per mile emission rates for particulate matter from exhaust and break and tire wear do not vary with speed in MOBILE6.

Figure 1: MOBILE6 Speed Correction Factors for Freeways (Tier 1 vehicles)

Source: Facility Specific Speed Correction Factors, Draft, U.S. EPA, Report Number M6.SPD.002, August 1999.

To account for the effects of speed, MOBILE6 calculates emission rates that are specific to each speed grouping (called "speed bin"). The speed bins are defined in 5 mph increments. When developing area-wide emissions estimates, users typically input the share of VMT that occurs at the different speed levels, and MOBILE6 then weights the speed-specific emission rates by VMT to produce a composite emission factor.

3.2 MOBILE6 Requirements for Speed

MOBILE6 accounts for speed effects by calculating emission rates specific to each speed, then weighting the speed-specific emission rates by VMT to produce a composite emission factor (by facility type). Thus, when developing area-wide emissions estimates, users have the option of inputting the fraction of VMT that occurs at the different speed levels, or rely on the model default values for the distribution of VMT by speed. The necessary level of detail of speed information supplied by the model user depends on local conditions and the intended uses of the emissions estimates (e.g., area-wide inventory vs. photochemical modeling).

VMT Distribution by Speed and by Hour (most detailed)

The greatest level of detail a user can provide for speed information is to specify the distribution of VMT by speed and by hour of day. This is accomplished using the SPEED VMT command in MOBILE6. For each hour of the day, the user provides the fraction of VMT that occurs in each of 14 speed "bins," shown in Table 1.

The distribution of VMT by speed will vary by roadway functional class. The four functional classes in MOBILE6 are as follows:

• Freeway (excluding ramps)
• Arterial/collector
• Local roadway
• Freeway Ramp

MOBILE6 allows users to enter a distribution of VMT by speed only for freeways and arterials/collectors. For local roadways and freeway ramps, the average speed in the model is fixed at 12.9 mph and 34.6 mph, respectively, and cannot be modified. Thus, a MOBILE6 user that provides the most detailed local speed information possible will input a 24 x 14 x 2 matrix of VMT fractions (24 hours in the day, 14 speed bins, 2 facility types). EPA recommends that local estimates at this level of detail be developed for non-attainment areas that are classified as moderate or above and that are required to perform photochemical modeling. Most small urban and rural areas are not expected to provide this level of detail, and most are unlikely to have the data necessary to develop such inputs.

VMT Distribution by Speed

A simpler option is to provide localized data for VMT fractions by speed bin for an entire 24-hour period (i.e., no local information on variation by hour of day). For the development of an on-road emission inventory or forecast for SIP or conformity purposes, EPA expects nonattainment areas that are classified as moderate or above to use their own such specific estimates of VMT by average speed.^[11]

This option might be used, for example, by a region that runs a travel demand forecasting (TDF) model for a 24-hour period, but has little data on how the VMT distribution by speed varies by hour of the day. The TDF model will provide the VMT and speed on every modeled link. Using the modeled speed (or some other derivation of speed), the VMT on each link is assigned to one of the 14 speed bins (separately for freeways and for arterials/collectors). The VMT in each speed bin is then divided by the sum of all the VMT to calculate the VMT fractions.

As an illustration, Table 2 shows the VMT and VMT fractions by speed bin for freeways and arterials used for a 1995 inventory for Ada County, Idaho. In this example, most freeway VMT occurs in the 47.5 mph to 62.5 mph range and most arterial VMT occurs in the 27.5 mph to 37.5 mph range.

Source: Guidance for the Development of Facility Type VMT and Speed Distributions, U.S. EPA, Report Number M6.SPD.004, February 1999.

MOBILE6 still requires users to input a VMT distribution by speed for each hour of the day in order to use the SPEED VMT command. If the region does not have hourly-specific speed data (i.e., speeds reflect a daily average or peak and off-peak), then the same values may be used for multiple hours in order to make a complete set of hourly values for the MOBILE6 input file.^[12] Alternatively, a user could run MOBILE separately for each speed bin and facility type using average speeds (as described below), then calculate a composite emission factor outside the model (e.g., in a spreadsheet).

Average Speed

If a region has no acceptable information regarding the distribution of VMT by speed, but does have some local information regarding the average speed by facility type, then MOBILE6 allows users to enter a single average speed for the freeway and arterial/collector functional classes using the AVERAGE SPEED command. (Local road and freeway ramp speeds are fixed in the model.) Doing this will bypass the model default speed distribution. For example, if a user enters 50 mph as the freeway speed, then all VMT on all freeway links are assumed to be 50 mph. Because the effect of speed on emission rates is not linear, using such a single average speed would produce different emission results than using a VMT distribution across speeds that averages 50 mph.

MOBILE Default Distributions

Finally, users can rely entirely on MOBILE defaults for speed information. MOBILE6 will calculate emission factors based on a default distribution of VMT by speed bin and by hour. This default distribution is intended to be a national distribution representing all Federal-aid urbanized areas (areas with a population of 50,000 or more). It was developed using data from Chicago, Houston, Charlotte, Ada County (Boise, ID) and New York City regions. Thus, the MOBILE6 default distribution of VMT by speed bin may not be appropriate for small urban and rural areas.

For example, Table 3 shows the MOBILE6 default VMT distribution by speed at 5 pm. (The model has similar distributions for every hour of the day.) More than 30 percent of freeway VMT in this distribution occurs at speeds less than 42.5 mph, indicating significant congestion. This distribution would not be representative of a rural area with little peak-period freeway congestion.

Source: Development of Methodology for Estimating VMT Weighting by Facility Type, U.S. EPA, Report Number M6.SPD.003, April 2001.

Use of Local Speed Data

Many of the methods described in this section require collection and processing of local data on observed speeds. It is important that users understand how to interpret specific speed databases because of the variety of methods used to report speeds. For example, pairs of in-road loop detectors provide instantaneous speeds of individual vehicles at a fixed location that may not be representative of overall speed distributions along the roadway segment. Average speeds may be calculated, as either arithmetic means or harmonic means (the inverse of the average of the inverses of observed speeds). For speed measurements at a specific location, these are sometimes referred to, respectively, as "time-mean speed" and "space-mean speed." The harmonic mean is generally preferred because the arithmetic mean provides a positively biased estimate. Arithmetic means from some other measurement methods (e.g., second-by-second recording in instrumented vehicles) can be used to provide unbiased space-mean speeds.^[13]