Sample Methodologies for Regional Emissions Analysis in Small Urban and Rural Areas

Estimating Speed without a TDF Model

Method 1: Use Observed Speeds and/or Speed Limits

Description

This methodology makes assumptions about average speeds by facility type based on posted speed limits and/or observed speeds. No analysis of link-specific traffic volumes or speeds is involved.

Method Applicability

This method is most applicable for areas that lack roadway volume and capacity information. This method would be most appropriate for rural areas that expect to see no change in travel speeds over time. Some areas use this method to estimate average speed on a particular facility type (such as collectors and local roads) and use more accurate methods to estimate speeds on other facility types.

Data Sources and Procedures

Speed Estimation

A single average speed is estimated for each facility type, based on the posted speed limit, observed travel speeds, or professional judgment. No analysis of traffic volumes and capacity is used to determine speeds.

Because this method is relatively simplistic, it typically assumes no change in speed over time. Thus, the future year speeds by facility type are assumed to be identical to the base year speeds.

Advantages

• Simplicity of the approach.
• Requires little or no new data collection.
• Requires little effort by the analyst.
• If based on observed speeds, likely to be more accurate than using MOBILE default speed information.

Limitations

• Accuracy is likely to be poor because observed speed data will reflect only a portion of roadway segments.
• Does not account for effects of changes in VMT and congestion over time.

Example Location

The Colorado DOT assumed an average speed of 35 mph for all arterials and 25 mph for all collectors and local roads in Aspen for estimating PM-10 emissions in a conformity analysis conducted for State Highway 82 (Entrance to Aspen).

The Yuma MPO (Arizona) assumed at average speed of 10 mph for all unpaved roads for estimating PM-10 emissions in its 2003 conformity analysis.

References:

Colorado DOT, "Air Quality Analysis, A Technical Report to the State Highway 82 Entrance to Aspen Environmental Impact Statement," July 7, 1995.

Yuma MPO, "2003 Air Quality Conformity Procedures Outline", draft adopted June 2003.

Lima & Associates "Vehicle Particulate Emissions Analysis" prepared for ADOT, and Yuma MPO, May 2002.

Estimating Speed without a TDF Model

Method 2: Use HERS Model at Statewide Level

Description

This methodology makes use of the Highway Economic Requirements System (HERS) model to estimate speed. HERS is a computer model developed by FHWA to analyze the effects of alternative funding levels on highway performance. HERS uses HPMS data to analyze the benefits and costs of alternative improvements. HERS computes vehicle speeds for the purpose of determining link travel time and vehicle operating cost, and these speed estimates can be used for calculating emissions. The latest version of the HERS model has a much more accurate speed estimation methodology than the HPMS Analytical Process, which was used by FHWA until the mid-1990s.

The original HERS model was developed for use at national scale. The model can now be customized for use at the statewide level (called HERS-ST). For more information, see FHWA, HERS-ST v20: Highway Economic Requirements System - State Version Technical Report, November 17, 2003.

Available online at http://isddc.dot.gov/OLPFiles/FHWA/010945.pdf.

Method Applicability

Most applicable for states that desire statewide speed estimates for all rural roads, by functional class. Method does not produce regionally specific speed estimates. Method may not produce accurate results for urban roadway classes in small urban areas.

Data Sources and Procedures

Speed Estimation

The HERS model is set up and run for the entire state. HERS uses HPMS data as inputs. HPMS data includes only a sample of roadway segments, and the sample size is too small in most cases to produce valid results at the county-level. Thus, the model should be run with the intent of estimating speed by roadway functional class for the entire states.

The latest version of HERS (version 3.54) uses a simplified version of the "Aggregate Probabilistic Limiting Velocity Model" (APLVM) in order to calculate free-flow speeds on each link. The model then applies algorithms to incorporate the effects of grades, traffic-control devices, and congestion on vehicle speed.

For each roadway section, HERS models speed by vehicle type in each direction of travel. Overall average speed per section is aggregated from the speeds of the individual vehicle types. Average link speeds are a by-product of the HERS model rather than one of the standard outputs. Link average speeds are then grouped by the 12 HPMS functional classes (six urban and six rural) for the entire state and averaged by facility type.

Because HERS is designed to evaluate future investment scenarios, it estimates both current and future speeds, based on current and forecasted traffic volumes. If the HERS traffic volume forecasts are consistent with the information used to forecast VMT for emissions purposes (Section 2), then the HERS speed forecasts would be acceptable for emissions estimation purposes. If not, then an alternative method may be needed to estimate future speeds.

Advantages

• Uses existing HPMS data. Does not require new data collection.
• Speed calculation algorithm considered accurate.
• Accounts for future congestion impacts on speed.

Limitations

• Speeds may not be very sensitive to specific local/regional conditions.
• Running the HERS-ST model can require substantial set-up time if the state is not already using the model.

Example Location

Kentucky used this method to estimate statewide average speeds by functional class in rural areas. These values were used to estimate NO_x and VOC emissions in two isolated rural areas.

Website: http://transportation.ky.gov/planning/air_quality.asp

Reference:

Bostrom, Rob and Jesse Mayes, "Highway Speed Estimation for MOBILE6 in Kentucky," Kentucky Transportation Cabinet, 2002.

Estimating Speed without a TDF Model

Method 3: Use BPR Formula or Variation

Description

This methodology estimates speed using some form of the "BPR formula," which is based on the volume/capacity (V/C) ratio and the free-flow speed. The original BPR formula was developed in the 1960s. More recent modifications to the formula parameters can improve accuracy of speed estimates. For detailed information on the BPR formula and related methods, refer to NCHRP Report 387, Planning Techniques to Estimate Speeds and Service Volumes for Planning Applications and the Appendix.^[14]

Method Applicability

This method is applicable for regions that have volume and capacity data by roadway segment, or can accurately estimate aggregate volume and capacity by facility type. This method is appropriate for areas with roadway congestion that are estimating NO_x, VOC, or CO emissions.

Data Sources and Procedures

This method can be applied to determine speeds on individual links, which can then be used to estimate a VMT distribution by speed bin for MOBILE6 input. Alternatively, this method can be applied to average VMT and capacity values in order to determine an average speed by functional class.

BPR-type formulas require three inputs: free-flow speed, roadway capacity, and traffic volume. Traffic volume information is developed as described in Section 2 of this report. The accuracy of this method is highly dependent on the accuracy of the capacity and free-flow speed inputs. Procedures for developing these two inputs are described below as well as in the Appendix of this report, followed by procedures for applying the BPR formula.

Free-flow speed estimation

NCHRP Report 387 recommends estimating free-flow speed by link using the following separate equations for unsignalized and signalized facilities.

[Note: additional discussion of free flow speed estimation techniques can be found in the Transportation Research Board's 2000 Highway Capacity Manual.]

Free-flow speed equation for unsignalized facilities:

Free-flow speed = 0.88*S_p + 14

(High-speed facilities have posted speed>50 mph)

Free-flow speed = 0.79*S_p + 12

(Low-speed facilities have posted speed<=50 mph)

where S_p = posted speed limit in mph

Free-flow speed equation for signalized facilities:

where: L = length of facility (in miles)

S_mb = mid-block free-flow speed = 0.79*posted speed + 12 mph

N = number of signalized intersections on length, L

D = average delay per signal

D = DF * 0.5 * C(1-g/C)²

where: D = total signal delay per vehicle (sec)

g = effective green time (sec)

C = cycle length (sec)

(Default values for these parameters are provided in the Appendix.)

When using these equations to estimate free-flow speed on a large number of links, it is typically impractical to apply the equations individually for each link. Instead, the equations are used to develop look-up tables of free-flow speeds by facility type and area type. The look-up table is then used to quickly assign free-flow speeds to each link. The Appendix includes an example of such a look-up table.

Free-flow speeds can be determined using other more simplistic methods. Some regions estimate flow speeds by facility type based on the posted speed limit. In other cases, areas add or subtract a fixed amount to/from the speed limit (e.g., speed limit plus 5 mph for highways) or multiply the speed limit by a fixed percentage (e.g., 62% of speed limit for collectors). These simple adjustments to posted speed limits are usually based on a limited sample of measured local speeds that are available for the desired roadway classification. When using these rules for estimating free-flow speeds, the equations often differ based on area type (e.g., CBD, rural, etc.). Other regions estimate free-flow speeds by facility type using observations of off-peak speeds.

Roadway capacity estimation

NCHRP Report 387 recommends a set of equations for estimating capacity that are based on the 1994 Highway Capacity Manual. There are separate equations for freeways, 2-lane unsignalized roads, and signalized arterials.

[Note: a more detailed discussion of capacity estimation techniques can be found in the Transportation Research Board's 2000 Highway Capacity Manual.]

Capacity equation for freeways and unsignalized multilane roads:

Capacity (vph) = Ideal Cap * N * F_hv * PHF

Where: Ideal Cap = ideal capacity in passenger cars per hour per lane (pcphpl)

N = number of through lanes

F_hv = heavy vehicle adjustment factor

PHF = peak-hour factor

Capacity equation for two-lane unsignalized roads:

Capacity (vph) = Ideal Cap * N * F_w * F_hv * PHF * F_dir * F_nopass

Where: Ideal Cap = ideal capacity in pcphpl

N = number of through lanes

F_w =lane width and lateral clearance factor

F_hv = heavy vehicle adjustment factor

PHF = peak-hour factor

F_dir = directional adjustment factor

F_nopass = no-passing zone factor

Capacity equation for signalized arterials:

Capacity (vph) = Ideal Cap * N * F_hv * PHF * F_park * F_bay * F_CBD * g/C * F_c

Where: Ideal Cap = ideal capacity in pcphpl

N = number of through lanes

F_hv = heavy vehicle adjustment factor

PHF = peak-hour factor

F_park = on-street parking adjustment factor

F_bay = left turn bay adjustment factor

F_CBD = central business district adjustment factor

g/C = ratio of effective green time per cycle

F_c = optional user-specified calibration factor

The parameters to use in these equations are provided in the Appendix.

As with free-flow speeds, it is usually impractical to apply the capacity equations individually for every link, so look-up tables are developed.

Computing average speed

The updated BPR formula is as follows:

where: s = predicted mean speed

s_f = free-flow speed

v = volume

c = practical capacity

a = 0.05 for facilities with signals spaced 2 mi apart or less

= 0.20 for all other facilities

b = 10

Many regions have modified the parameters a and b so that the formula calculates speeds that more closely reflect observed local speeds. The original BPR formula uses a = 0.15 and b = 4. Other regions have used values of a as high as 1.0 and values of b as high as 11.

Advantages

• Able to produce highly accurate speed estimates if applied properly.
• Accounts for future congestion impacts on speed.

Limitations

• In order to produce accurate speed results, requires accurate local information on capacity and free-flow speed. Use of default look-up tables for these values can lead to inaccurate speed estimates.
• To apply this method for individual links requires detailed information regarding signalization characteristics, traffic characteristics, etc.
• Method not accurate for V/C ratios over 1.0.

Example Location

Ohio DOT used the original form of the BPR formula (a = 0.15 and b = 4) to estimate speed in rural areas not covered by a TDF model. To estimate free-flow speeds, Ohio DOT used the upper bound of the table provided in the HCM for each functional class.

Website: http://www.dot.state.oh.us/urban/index.htm

(Speed forecasting procedures described under "documents" section)

References:

Ohio DOT, "Technical Memorandum: Clinton County 2000-2003 STIP/TIP Emissions Estimate," May 25, 1999.

Estimating Speed without a TDF Model

Method 4: Use TTI Method

Description

This methodology estimates aggregate average speeds by functional class, time-of-day period, and direction. The methodology uses Highway Performance Monitoring System (HPMS) data, equations to calculate delay and congested speed, and look-up tables for parameters based on the 1994 HCM. For more information on this method, refer to Dresser, George B., and Dennis G. Perkinson, "Development of On-Road Mobile Source Emission," Texas Transportation Institute, Paper prepared for the 10th Annual Emission Inventory Conference: Inventories for Rural Counties, May 2001.

Method Applicability

This method is applicable for regions that have volume and capacity data by roadway segment, or can accurately estimate aggregate volume and capacity by facility type. This method is appropriate for areas with roadway congestion that are estimating NO_x, VOC, or CO emissions.

Data Sources and Procedures

This method can be applied to determine speeds on individual links, which can then be used to estimate a VMT distribution by speed bin for MOBILE6 input. Alternatively, this method can be applied to average VMT and capacity values in order to determine an average speed by functional class. The procedure described below is to determine average speed by functional class.

This method requires estimation of traffic volume, free-flow speed, and roadway capacity, using HPMS data aggregated by area type and functional class.

Volume estimation

HPMS data is first separated into area types and roadway functional classifications. Area type is defined by population - rural (4,999 or less), small urban (5,000 to 49,999), and urban (50,000+). Functional classifications are based on the HPMS classes (Interstate, freeway, other principal arterial, minor arterial, major collector, minor collector, and local).

VMT by area/functional class is allocated into time periods. The four default time periods correspond to the AM peak (7:15 a.m. - 8:15 a.m.), mid-day (8:15 a.m. - 4:45 p.m.), the PM peak (4:45 p.m. - 5:45 p.m.), and overnight (5:45 p.m. - 7:15 a.m.). The default allocation factors for these periods are, respectively, 0.1069, 0.5033, 0.1018, and 0.2880.

VMT by area/functional class and time period is further disaggregated by directional split. The default directional split is 60/40. VMT per time period is divided by centerline miles, yielding volume for each time period, each area type and functional class, and each direction.

Free-flow speed estimation

Free-flow speeds are estimated for each combination of area type and functional class. Default values are shown in the table below. Free-flow speeds are assumed not to vary by time-of-day period or direction.

Default values for free flow speed (mph)

HPMS Area Type	HPMS Roadway Functional Classification
	Interstate	Freeway	Other Principal Arterial	Minor Arterial	Major Collector	Minor Collector	Local
Rural	70	65	55	50	40	35	30
Small Urban	70	65	45	40	35	30	30
Urban	70	65	40	35	30	30	30

Roadway capacity estimation

Roadway capacity values are estimated based on the 1994 HCM. For all Interstates, the method uses a default capacity of 2,200 passenger cars per hour per lane (pcphpl). Freeways are assumed to have a default capacity of 2,100 pcphpl.

Other functional class roadways have traffic controls, so capacity is determined using the following equation:

C_i = S_i * (g_i/C)

Where: C_i = capacity of lane group I (vehicles per hour)

S_i = saturation flow rate of lane group i, vehicles per hour of effective green time (vphg)

g_i/C = effective green ratio for lane group i

Default values for effective green ratios (g_i/C) by HPMS roadway functional class

Principal Arterial	Minor Arterial	Major Collector	Minor Collector	Local
0.6	0.55	0.5	0.4	0.3

Saturation flow rate is calculated using the following equation:

S=f_w*f_hv*f_g*f_p*f_bb*f_a*f_rt*f_lt

Where: S = saturation flow rate adjustment factor (rounded to 2 decimal places)

f_w = lane width adjustment factor (default is 12-foot lanes)

f_hv = heavy vehicle adjustment factor (default is 5%)

f_g = approach grade factor (default is 1, level terrain)

f_p = parking lane adjustment (none for rural, 1 per hour for urban)

f_bb = bus blocking factor (none for rural, 10 per hour for urban, mid-point for small urban areas)

f_a = area type adjustment (0.9 for urban area, 1.0 for all other areas)

f_rt = right turn adjustment factor (shared lane for right turns for all area types, high pedestrians crossing for urban areas, moderate for small urban areas, and low for rural)

f_lt = left turn adjustment factor (exclusive left turn lanes and protected phasing for rural areas, shared left turn lanes and protected plus permitted phasing for urban areas, mid-point for small urban areas)

If possible, these parameters should be developed using local estimates, for each combination of area type and functional class. Otherwise, the method suggests use of the default values in the table below.

Default saturation flow rate adjustment factors by area type

Area Type	fw	fhv	fg	fp	fbb	fa	frt	flt
Rural	1	0.95	1	1	1	1	0.98	0.95
Small Urban	1	0.95	1	0.98	0.98	1	0.94	0.90
Urban	1	0.95	1	0.95	0.96	0.90	0.90	0.85

Using the default adjustment factors results in the following default values for hourly lane capacity, by area type and functional class.

Default hourly lane capacities (vehicles per hour per lane)

PMS Area Type	HPMS Roadway Functional Classification
	Interstate	Freeway	Other Principal Arterial	Minor Arterial	Major Collector	Minor Collector	Local
Rural	2,200	2,100	1,003	920	836	669	502
Small Urban	2,200	2,100	878	805	732	585	439
Urban	2,200	2,100	673	617	561	448	336

The hourly lane capacity values are then used to estimate aggregate capacity by time-of-day period. To do this, the lane capacities are multiplied by the number of lanes associated with each area type/functional class (lane miles divided by centerline miles). Hourly roadway capacities are then typically multiplied by the number of hours in the time period to produce time period capacities. This procedure is performed for each combination of time period, roadway functional classification, and area type. (Capacity is the same for each direction and time period.)

Computing average speed

Calculation of average speed requires the aggregate volume and capacity as described above and the free-flow speed values. The calculation of speed uses a formula originally developed by the North Central Texas Council of Governments for the Dallas/Fort Worth area. The procedures involves calculating delay using the following equation:

Where: Delay = congestion delay (in minutes/mile);

A & B = volume/delay equation coefficients;

M = maximum minutes of delay per mile; and

V/C = time-of-day directional volume/capacity ratio.

Parameters:

Facility Category	Parameter
	A	B	M
Facilities (>3,400 vehicles/hour, e.g. Interstates and Freeways)	0.015	3.5	5
Low Capacity Facilities (<3,400 vehicles/hour, e.g. Arterials, Collectors, and Locals)	0.05	3	10

Congested speeds can then be calculated as follows:

The result is an estimate of average speed for each combination of area type, functional class, and time-of-day period. These values can then be used to determine VMT distributions by speed for the MOBILE6 inputs.

Advantages

• Can produce highly accurate speed estimates.
• Accounts for future congestion impacts on speed.
• If using the default look-up tables for free-flow speed and capacity, this method can be applied relatively quickly, without major new data collection.

Limitations

• In order to produce accurate speed results, requires accurate information on capacity and free-flow speed.
• To apply this method for individual links, requires detailed information regarding signalization, traffic characteristics, etc.

Example Location

North Carolina used this method to estimate VOC and NO_x emissions in donut areas (part of a metropolitan non-attainment area but outside MPO boundaries). North Carolina used this method to estimate average speeds by area type (rural, small urban, urban) and functional class. North Carolina also tested the BPR formula and another speed formula (the Greenshields method), but found that the TTI method produced speed estimates that most closely match observed values.

References:

North Carolina DOT, Davidson County, and the North Carolina Department of Environment, "Emissions Analysis Report for the Transportation Plan for Rural Portion of Davidson County," September 27, 2002.

Rural Conformity Spreadsheet PowerPoint Presentation, Behshad Norowzi, North Carolina Department of Transportation, July 2004.