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 |
 |
|
|
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 |
|
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 |
|
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 |
|
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 |
|
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 |
|
miles |
0.93 |
0.88 |
6.0% |
| Total |
|
|
|
|
|
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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