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Guide for Highway Capacity and Operations Analysis of Active Transportation and Demand Management Strategies

Appendix A: Speed/Capacity for Weather

Two sources were used to obtain default capacity and free-flow speed adjustment factors for weather that are applicable to freeways. Exhibit 10-15 of the 2010 Highway Capacity Manual (shown in Table 33) provides capacity reductions. Exhibit 36-25 of the SHRP 2-L08 Final Report Draft HCM Chapters (shown in Table 34) provides both capacity and free-flow speed adjustments. Note that 1.00 minus the percent reduction equals the adjustment factor.

The HCM provides capacity adjustments for a slightly greater range of weather conditions than SHRP 2-L08 (Table 34), however; the added weather conditions tend to be ones with marginal effects on freeway capacity. The SHRP 2-L08 capacity adjustments are sensitive to free-flow speed (under fair weather), while the HCM adjustments are not. The SHRP 2-L08 exhibit also provides freeway free-flow speed adjustments for weather.

The HCM capacity reductions and the SHRP 2-L08 capacity adjustments generally match for freeways with 65 mph free-flow speeds. Consequently, the HCM capacity reductions (after conversion to the equivalent capacity adjustment factors) were used in combination with the SHRP 2-L08 free-flow speed adjustments (selected for 65 mph free-flow speed) for the example problem. Where the SHRP 2-L08 speed adjustments were lacking, interpolations of extrapolations of the factors were used. The final selected adjustments for the ATDM example problem are shown in Table 36.

Table 33: HCM Freeway Capacity Reductions for Weather
Type of Condition Intensity of Condition Average % Reduction Range of % Reductions
Rain >0 ≤ 0.10 in./h 2.01 1.17-;3.43
Rain >0.10 ≤ 0.25 in./h 7.24 5.67-;10.10
Rain >0.25 in./h 14.13 10.72-;17.67
Snow >0 ≤ 0.05 in./h 4.29 3.44-;5.51
Snow >0.05 ≤ 0.10 in./h 8.66 5.48-;11.53
Snow >0.10 ≤ 0.50 in./h 11.04 7.45-;13.35
Snow >0.50 in./h 22.43 19.53-;27.82
Temperature <50°F ≥ 34°F 1.07 1.06-;1.08
Temperature <34°F ≥ -4°F 1.50 1.48-;1.52
Temperature <-;4°F 8.45 6.62-;10.27
Wind >10 ≤ 20 mi/h 1.07 0.73-;1.41
Wind >20 mi/h 1.47 0.74-;2.19
Visibility <1 ≥ 0.50 mi 9.67 One site
Visibility <0.50 ≤ 0.25 mi 11.67 One site
Visibility <0.25 mi 10.49 One site

Source: Exhibit 10-15, 2010 Highway Capacity Manual. Entries are percent reduction in capacity.

Table 34: SHRP 2-L08 Freeway Capacity and Speed Adjustments for Weather
Weather Type Capacity Adjustment Factors – 55 mi/h Capacity Adjustment Factors – 60 mi/h Capacity Adjustment Factors – 65 mi/h Capacity Adjustment Factors – 70 mi/h Capacity Adjustment Factors – 75 mi/h Speed Adjustment Factors – 55 mi/h Speed Adjustment Factors – 60 mi/h Speed Adjustment Factors – 65 mi/h Speed Adjustment Factors – 70 mi/h Speed Adjustment Factors – 75 mi/h
Medium rain 0.94 0.93 0.92 0.91 0.90 0.96 0.95 0.94 0.93 0.93
Heavy rain 0.89 0.88 0.86 0.84 0.82 0.94 0.93 0.93 0.92 0.91
Light snow 0.97 0.96 0.96 0.95 0.95 0.94 0.92 0.89 0.87 0.84
Light-medium snow 0.95 0.94 0.92 0.90 0.88 0.92 0.90 0.88 0.86 0.83
Medium-heavy snow 0.93 0.91 0.90 0.88 0.87 0.90 0.88 0.86 0.84 0.82
Heavy snow 0.80 0.78 0.76 0.74 0.72 0.88 0.86 0.85 0.83 0.81
Severe cold 0.93 0.92 0.92 0.91 0.90 0.95 0.95 0.94 0.93 0.92
Low visibility 0.90 0.90 0.90 0.90 0.90 0.96 0.95 0.94 0.94 0.93
Very low visibility 0.88 0.88 0.88 0.88 0.88 0.95 0.94 0.93 0.92 0.91
Minimal visibility 0.90 0.90 0.90 0.90 0.90 0.95 0.94 0.93 0.92 0.91
Nonsevere weather 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Note: Speeds given in column heads are free-flow speeds. Entries are adjustment factors, not percent reductions.

With SHRP 2-L08 weather types as defined in Table 34.

Table 35: SHRP 2-L08 Weather Type Definitions for Freeways
Weather Event Definition
Medium rain >0.10 ≤ 0.25 in./h
Heavy rain >0.25 in./h
Light snow >0 ≤ 0.05 in./h
Light-medium snow >0.05 ≤ 0.10 in./h
Medium-heavy snow >0.10 ≤ 0.50 in./h
Heavy snow >0.50 in./h
Severe cold <–4˚F
Low visibility <1 ≥ 0.50 mi
Very low visibility <0.50 ≤ 0.25 mi
Minimal visibility <0.25 mi
Nonsevere weather All other conditions not listed above
Table 36: Selected Weather Adjustments for ATDM Example Problem
Weather Type Range Range Range Speed Adjustment Factor Capacity Adjustment Factor Probability
Clear Empty cell. Empty cell. Empty cell. 1.00 1.00 50.0%
Light Rain > 0.00 <= 0.10 in/hr 0.98 0.98 8.0%
Medium Rain > 0.10 <= 0.25 in/hr 0.94 0.93 4.0%
Heavy Rain > 0.25 Empty cell. in/hr 0.93 0.86 2.0%
Very Light Snow > 0.00 <= 0.05 in/hr 0.89 0.96 6.0%
Light Snow > 0.05 <= 0.10 in/hr 0.88 0.91 3.0%
Medium Snow > 0.10 <= 0.50 in/hr 0.86 0.89 2.0%
Heavy Snow > 0.50 Empty cell. in/hr 0.85 0.76 2.0%
Low Wind > 10.00 <= 20.00 mph 0.99 0.99 4.0%
High Wind > 20.00 Empty cell. mph 0.98 0.98 2.0%
Cool < 50.00 >= 34.00 degrees fahrenheit 0.99 0.99 2.0%
Cold < 34.00 >= -4.00 degrees fahrenheit 0.98 0.98 2.0%
Very Cold < -4.00 Empty cell. degrees fahrenheit 0.94 0.91 3.0%
Medium Visibility < 1.00 >= 0.50 miles 0.94 0.90 2.0%
Low Visibility < 0.50 >= 0.25 miles 0.93 0.88 2.0%
Very Low Visibility < 0.25 Empty cell. miles 0.93 0.88 6.0%
Total Empty cell. Empty cell. Empty cell. Empty cell. Empty cell. 100.0%

Source: 2010 Highway Capacity Manual (Transportation Research Board, 2010) and SHRP 2-L08 Final Report (Vandehey, Ryus, Bonneson, Rouphail, Margiotta, & Dowling, 2013).

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