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5 Summary

Small urban and rural areas that are required to conduct conformity analysis typically face many challenges in conducting the regional emissions analysis: limited data on VMT and speeds, lack of a travel demand forecasting (TDF) model, and often limited staff expertise in emissions modeling. While large metropolitan areas generally have advantages in terms of resources, data, and tools, it is important to recognize that valid methods are available to conduct the regional emissions analysis in small urban and rural areas.

This document provides a sampling of methodologies and adjustment techniques for developing estimates of VMT and speeds, the two key inputs required for emissions modeling. It also describes other factors that influence emissions factors, and highlights approaches for using local data rather than MOBILE6 model defaults. Each option has certain advantages and limitations. There is no "one-size fits all" approach that should be applied in all areas. Areas subject to conformity should identify what methods are most appropriate to their region through the interagency consultation process. The selected methods should reflect data availability and local conditions. Other options beyond those profiled in this report may also be available, and regions should explore other possibilities.

6 Resources

Resources on Regional Emissions Analysis Methodologies

2000 Highway Capacity Manual, Transportation Research Board, Washington D.C., 2000.

Analysis of Traffic Growth Rates, M.L. Barrett, R.C. Graves, D.L. Allen, J.G. Pigman, G. Abu-Lebdeh, L. Aultman-Hall, S.T. Bowling, University of Kentucky Transportation Center, August 2001.

Development of On-Road Mobile Source Emission Inventories for Rural Counties, G. B. Dresser, D. G. Perkinson, Texas Transportation Institute, May 2001. http://www.epa.gov/ttn/chief/conference/ei10/index.html#ses-6

HERS-ST v20: Highway Economic Requirements System - State Version Technical Report, FWHA, November 17, 2003. Online at: http://isddc.dot.gov/OLPFiles/FHWA/010945.pdf.

Highway Speed Estimation for MOBILE6, Bob Bostrom and Jesse Mayes, Kentucky Transportation Cabinet, 2002.

Planning Techniques to Estimate Speeds and Service Volumes for Planning Applications, NCHRP Report 387, Transportation Research Board, Dowling, R.; Kittelson, W; Zegeer, J.; Skabardonis, A, 1997.

Rural Conformity: A Survey of Practice, NCHRP, Project 08-36, Task 28, ICF Consulting and Sarah J. Siwek & Associates, October 2003.

Vehicle Miles of Travel Projections and Speed Estimates for Rural Nonattainment and Maintenance Areas, John Gardner, South Carolina Department of Transportation, presented at Southern Transportation and Air Quality Summit, October 2001, Atlanta, Ga.

General Resources on Conformity Requirements, Guidance, and Training

Conformity Requirements: Title 40 of the Code of Federal Regulations (40 CFR Parts 51 and 93) http://www.access.gpo.gov/nara/cfr/waisidx_99/40cfr93_99.html

Transportation Conformity: A Basic Guide for State and Local Officials, U.S. Federal Highway Administration, Revised June 19, 2000. http://www.fhwa.dot.gov/environment/conformity/con_bas.htm

Transportation Conformity Reference Guide. Federal Highway Administration (last updated July, 2001) http://www.fhwa.dot.gov/environment/conformity/ref_guid/index.htm

Clean Air Act (42 USC 7401-7671q) http://www.epa.gov/oar/caa/contents.html

SIP Requirements http://www.epa.gov/oar/caa/contents.html

National Transit Institute (NTI) Course on Introduction to Transportation/Air Quality Conformity: http://www.ntionline.com

National Highway Institute (NHI) Course on Estimating Regional Mobile Source Emissions

http://www.nhi.fhwa.dot.gov

Other FHWA/EPA Conformity Resources, including Transportation Conformity Community of Practice: http://www.fhwa.dot.gov/environment/conform.htm

Appendix
Parameters and Defaults Values for Use with the BPR Formula for Estimating Speed

As described in Section 3, use of the BPR-type formulas (and other methods) requires 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. This appendix described in detail the procedures for developing these two inputs, including default parameter values and some examples.

Free-flow speed estimation

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

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)

If signal timing data are not available, the following default values can

be used:

C = 120 seconds

g/C = 0.45

DF = (1 - P)/(1 - g/C), where P= proportion of vehicles arriving on green

If P is unknown, the following defaults can be used for DF:

DF = 0.9 for uncoordinated traffic actuated signals

= 1.0 for uncoordinated fixed time signals

= 1.2 for coordinated signals with unfavorable progression

= 0.90 for coordinated signals with favorable progression

= 0.60 for coordinated signals with highly favorable progression

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. Below is an example if such a look-up table.

Example - Look-up table of default free-flow speeds (mph)

	Free-flow speeds (mph)
	Freeway	Expressway	Arterial	Collector	Local
CBD	50	45	40	35	30
Urban	55	50	45	40	35
Suburban	60	55	50	45	40
Rural	65	60	55	50	45

Source: Planning Techniques to Estimate Speeds and Service Volumes for Planning Applications,

NCHRP Report 387, Transportation Research Board, 1997.

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, such as adding or subtracting 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 observed 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.

Capacity equation for freeways and unsignalized multilane roads:

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

Where:

Ideal Cap = 2,400 (pcphl) for freeways with >=70 mph free-flow speed

= 2,300 (pcphl) for all other freeways (free-flow speed < 70 mph)

N = number of through lanes (Ignore auxiliary lanes and "exit only" lanes)

F_hv = heavy vehicle adjustment factor

= 100/(100 + 0.5 * HV) for level terrain

= 100/(100 + 2.0 * HV) for rolling terrain

= 100/(100 + 5.0 * HV) for mountainous terrain

(HV = proportion of heavy vehicles, including trucks, buses, recreational vehicles, in the traffic flow. If HV is unknown, use 0.05 heavy vehicles as default.)

PHF = peak-hour factor (ratio of the peak 15-min flow rate to the average hourly flow rate) (If unknown, use default value of 0.90.)

Capacity equation for two-lane unsignalized roads:

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

Where: Ideal Cap = 1,400 (pcphl) for all two-lane rural roads

N = number of lanes

F_w = lane width and lateral clearance factor

= 0.80 if narrow land and/or narrow shoulders are present

= 1.00 otherwise

(Narrow lanes are less than 12 ft. (3.6 m) wide; narrow shoulders are less than 3 ft (1.0 m) wide.)

F_hv = heavy vehicle adjustment factor

= 100/(100 + 1.0 * HV) for level terrain

= 100/(100 + 4.0 * HV) for rolling terrain

= 100/(100 + 11.0 * HV) for mountainous terrain

(HV = proportion of heavy vehicles, including trucks, buses, recreational vehicles, in the traffic flow. If HV is unknown, use 0.05 heavy vehicles as default.)

PHF = peak-hour factor (ratio of the peak 15-min flow rate to the average hourly flow rate) (If unknown, use default value of 0.90.)

F_dir = directional adjustment factor

= 0.71 + 0.58 * (1.00 - peak direction proportion) (Peak direction proportion of two-way traffic going in peak direction. If not known, use default of 0.55 peak direction.)

F_nopass = no-passing zone factor

= 0.97 - 0.07 * (NoPass) for rolling terrain

= 0.91 - 0.13 * (NoPass) for mountainous terrain

(NoPass is the proportion of length of facility for which passing is prohibited. If NoPass is unknown, use 0.60 NoPass for rolling terrain and 0.80 for mountainous terrain.)

Capacity equation for signalized arterials:

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

Where: Ideal Sat = ideal saturation flow rate (vehicles per lane per hour of green)

= 1,900

N = number of through lanes (Exclude exclusive turn lanes and short lane additions.)

F_hv = heavy vehicle adjustment factor

= 1.00/(1.00 + HV)

(HV = proportion of heavy vehicles, including trucks, buses, recreational vehicles, in the traffic flow. If HV is unknown, use 0.05 heavy vehicles as default.)

PHF = peak-hour factor (ratio of the peak 15-min flow rate to the average hourly flow rate) (If unknown, use default value of 0.90.)

F_park = on-street parking adjustment factor

= 0.90 if on-street parking is present and time limit is 1 hr or less

= 1.00 otherwise

F_bay = left turn bay adjustment factor

= 1.10 if exclusive left turn lanes (often as left turn bay) are present

= 1.00 otherwise

F_CBD = central business district adjustment factor

= 0.90 if located in CBDs

= 1.00 elsewhere

g/C = ratio of effective green time per cycle

If no data are available, use the following defaults:

Protected left turn phase present: g/C = 0.40

Protected left turn phase not present: g/C = 0.45

Other defaults may be developed by the local planning agency based on local conditions. Additional defaults might be based on the functional class of major and crossing streets.

F_c = optional user-specified calibration factor necessary to match estimated capacity with field measurements or other independent estimates of capacity (no units) (can be used to account for the capacity-reducing effects of left and right turns made from through lanes)

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

Example - Look-up table of practical capacity for original BPR curve

	One-way LOS "C", vehicles per lane per hour
	Freeway	Expressway	2-Way Arterial (w/ parking)	One-Way Arterial (w/ parking)	Two-Way Arterial (no parking)
CBD	1750	800	600	700	600
Fringe	1750	1000	550	550	800
Outer CBD	1750	1000	550	650	800
Rural/Residential	1750	1000	550	900	800

Source: Planning Techniques to Estimate Speeds and Service Volumes for Planning Applications, NCHRP Report 387, Transportation Research Board, 1997.

If traffic volume data is on a daily basis (AADT), then hourly capacity must be converted to an effective daily capacity. In one approach to calculate 24-hour capacity, the hourly capacity per lane is divided by the ratio of AADT that occurs in the peak hour. This figure is then multiplied by the number of lanes in the peak direction, and in the off peak direction is multiplied by the number of lanes and a directional adjustment factor. A 24-hour volume-to-capacity (V/C) ratio is then calculated by dividing AADT by 24-hour capacity.

Construction of a Localized Capacity Look-Up Table

Because the accuracy of capacity estimates is essential to the accuracy of speed estimates, NCHRP Report 387 recommends that planning agencies use the specific capacities of the selected study section. When that is not possible, the following tables demonstrate the procedure for selecting default values and computing a look-up table of capacities, according to facility, area, and terrain type. The first table is for two-lane, rural undivided arterials, but additional rows of data could be added for multilane rural undivided arterials. The second table provides a sample computation.

Example - Table for Entering Default Values for Computing Capacity by Functional Class and Area/Terrain Type

Functional Class	Area Type	Terrain Type	Lanes	Free Speed	Lane Width	PHF	% Heavy Vehicles	Direction Split	% No Pass	Parking	Left Turn Bay	g/C
Freeway	Rural	Level	all	>70 mph		0.85	5%
		Rolling	all	>70 mph		0.85	5%
		Mountain	all	<70 mph		0.85	5%
	Urban	all	all	<70 mph		0.90	2%
Divided Arterial	Rural	Level	>2	60 mph		0.85	5%
		Rolling	>2	55 mph		0.85	5%
		Mountain	>2	50 mph		0.85	5%
	Suburban	all	all			0.90	2%			no	yes	0.45
	Urban	all	all			0.90	2%			yes	yes	0.45
	CBD	all	all			0.90	2%			yes	yes	0.45
Undivided Arterial	Rural	Level	2		standard	0.85	5%	55%	0%
		Rolling	2		standard	0.85	5%	55%	60%
		Mountain	2		narrow	0.85	5%	55%	80%
	Suburban	all	all			0.90	2%			no	no	0.45
	Urban	all	all			0.90	2%			yes	no	0.45
	CBD	all	all			0.90	2%			yes	no	0.45
Collector	Urban	all	all			0.85	2%			yes	no	0.40

Example - Computation of Default Capacities by Functional Class and Area/Terrain Type

Functional Class	Area Type	Terrain Type	Lanes	Ideal Cap	PHF	Fhv	FW	Fdir	Fno- pass	Fpark	Fleft	Fcbd	g/C	Cap/ Lane
Freeway	Rural	Level	all	2400	0.85	0.98								2000
		Rolling	all	2400	0.85	0.91								1900
		Mountain	all	2300	0.85	0.80								1600
	Urban	all	all	2300	0.90	0.98								2000
Divided Arterial	Rural	Level	>2	2200	0.85	0.98								1800
		Rolling	>2	2100	0.85	0.91								1600
		Mountain	>2	2000	0.95	0.80								1400
	Suburban	all	all	1900	0.90	0.98				1.00	1.10	1.00	0.45	850
	Urban	all	all	1900	0.90	0.98				0.90	1.10	1.00	0.45	750
	CBD	all	all	1900	0.90	0.98				0.90	1.10	0.90	0.45	650
Undivided Arterial	Rural	Level	2	1400	0.85	0.95	1.00	0.97	1.00					1100
		Rolling	2	1400	0.85	0.83	1.00	0.97	0.93					900
		Mountain	2	1400	0.85	0.65	0.80	0.97	0.81					500
	Suburban	all	all	1900	0.90	0.98				1.00	1.00	1.00	0.45	750
	Urban	all	all	1900	0.90	0.98				0.90	1.00	1.00	0.45	700
	CBD	all	all	1900	0.90	0.98				0.90	1.00	0.90	0.45	600
Collector	Urban	all	all	1900	0.85	0.98				0.90	1.00	1.00	0.40	550

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 often leads to inaccurate speed estimates.
• To apply this method for individual links, requires detailed information regarding signalization characteristics, traffic characteristics, etc.

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.

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^[1] In California, emissions factors are developed through the EMFAC model rather than through MOBILE. Although the MOBILE model is referred to through this report, many of the same methodologies can also be applied in developing inputs to EMFAC.

^[2] A donut area is a geographic area that falls within the boundary of a nonattainment or maintenance area that contains a metropolitan area, but falls outside of the metropolitan planning area boundary. Emissions in donut areas in most cases must be included in the metropolitan area regional emissions analysis for the Plan and TIP.

^[3] It should be noted that EPA's MOBILE6 model is currently required for use in conformity analysis for all states outside of California. Several of the methodologies identified through this research were applied using previous versions of EPA's MOBILE model, EPA's PART5 model to estimate particulate matter, or the EMFAC model in California. These methodology descriptions, in some cases, have been adapted slightly to reflect procedures that could be applied using MOBILE6.

^[4] The HPMS provides data that reflects the extent, condition, performance, use, and operating characteristics of the nation's highways. For more information on background, scope, major uses of the HPMS, and reporting requirements, consult FHWA's HPMS Field Manual at http://www.fhwa.dot.gov/ohim/hpmsmanl/hpms.cfm.

^[5] Adapted from Guidance for the Development of Facility Type VMT and Speed Distributions, U.S. EPA, Report Number M6.SPD.004, February 1999.

^[6] Methods for estimating VMT by speed bin are discussed in Section 3. Methods for estimating VMT mix by vehicle type are discussed in Section 4.2.

^[7] The MOBILE6 emissions model functions differently than MOBILE5, which simply developed emission factors based on average speeds. As a result, methodologies for conformity analysis using MOBILE5 did not need to make this distinction between the different definitions of local roadways, and commonly developed emissions estimates based on average speed by functional roadway classification.

^[8] This formula is nearly equivalent to method 1, except that it allows for a certain baseline VMT level on local roads that is independent of the volume on collector roads.

^[9] American Association of State Highway and Transportation Officials, A Policy on the Design of Highways and Streets [2001 Greenbook], 2001.

^[10] A REMI model (Regional Economic Models, Inc.) is a commonly used economic-forecasting and policy-analysis model;

^[11] Technical Guidance on the Use of MOBILE6.2 for Emission Inventory Preparation, U.S. EPA, August 2004.

^[12] Technical Guidance on the Use of MOBILE6.2 for Emission Inventory Preparation, U.S. EPA, August 2004.

^[13] This paragraph adapted from Guidance for the Development of Facility Type VMT and Speed Distributions, U.S. EPA, Report Number M6.SPD.004, February 1999.

^[14] Additional detail on BPR formulas and related methods can be found in the Transportation Research Board's 2000 Highway Capacity Manual.

^[15] USEPA, 2002. "Sensitivity Analysis of MOBILE6.0", Assessment and Standards Division, Office of Transportation and Air Quality, EPA420-R-02-035, December 2002.

^[16] Evaluating Vehicle Emissions Inspection and Maintenance Programs, Committee on Vehicle Emission Inspection and Maintenance Programs, Transportation Research Board, National Research Council, National Academy of Press, 2001. ISBN 0-309-07446-0.