15.4.4.2 Weather Events

The base scenario assumes ideal driving weather conditions; clear, no precipitation, and high visibility. However, different weather conditions have a wide range of impacts on the capacity of a freeway and the free flow travel speed, depending on the severity of the weather event. Inclement weather, such as snow, rain, or fog, can degrade driving conditions by creating slick roads, flooding, or limiting visibility. These conditions may result in reduced roadway capacity and reduced free flow speeds.
CAFs and SAFs for weather events are applied to scenarios across a RRP based on the probability of a weather event’s occurrence during a given month in the freeway location. For example, if historical weather trends indicate for a given month that heavy rain events occur across ten percent of days, then the relevant AFs for that weather event can be applied at random to ten percent of the scenarios analyzed for that month. This methodology can be repeated for all types of weather. The 11 weather categories consist of one non-severe weather category that includes all weather conditions that generate no capacity, demand, or speed adjustments, and ten categories of severe weather events that have been shown to reduce capacity by at least four percent. CAFs are shown in and SAFs are shown in .
Table 15-2: Default Urban Freeway Demand Ratios (ADT/Mondays in January)
Month
Day of Week
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
January
1.00
1.00
1.02
1.05
1.17
1.01
0.89
February
1.03
1.03
1.05
1.08
1.21
1.04
0.92
March
1.12
1.12
1.14
1.18
1.31
1.13
0.99
April
1.19
1.19
1.21
1.25
1.39
1.20
1.05
May
1.18
1.18
1.21
1.24
1.39
1.20
1.05
June
1.24
1.24
1.27
1.31
1.46
1.26
1.10
July
1.38
1.38
1.41
1.45
1.62
1.39
1.22
August
1.26
1.26
1.28
1.32
1.47
1.27
1.12
September
1.29
1.29
1.32
1.36
1.52
1.31
1.15
October
1.21
1.21
1.24
1.27
1.42
1.22
1.07
November
1.21
1.21
1.24
1.27
1.42
1.22
1.07
December
1.19
1.19
1.21
1.25
1.40
1.20
1.06
Note: Ratios represent demand relative to a Monday in January
Source: HCM 7th Edition
Table 15-3: Default Rural Freeway Demand Ratios (ADT/Mondays in January)
Month
Day of Week
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
January
1.00
1.96
1.98
1.03
1.22
1.11
1.06
February
1.11
1.06
1.09
1.14
1.35
1.23
1.18
March
1.24
1.19
1.21
1.28
1.51
1.37
1.32
April
1.33
1.27
1.30
1.37
1.62
1.47
1.41
May
1.46
1.39
1.42
1.50
1.78
1.61
1.55
June
1.48
1.42
1.45
1.53
1.81
1.63
1.57
July
1.66
1.59
1.63
1.72
2.03
1.84
1.77
August
1.52
1.46
1.49
1.57
1.86
1.68
1.62
September
1.46
1.39
1.42
1.50
1.78
1.61
1.55
October
1.33
1.28
1.31
1.38
1.63
1.47
1.42
November
1.30
1.25
1.28
1.35
1.59
1.44
1.39
December
1.17
1.12
1.14
1.20
1.43
1.29
1.24
Note: Ratios represent demand relative to a Monday in January
Source: HCM 7th Edition
Table 15-4: Default CAFs by Weather Condition
Weather Type
Weather Event Definition
Capacity Adjustment Factors
55 mi/h
60 mi/h
65 mi/h
70 mi/h
75 mi/h
Medium rain
>0.10-0.25 in./h
0.94
0.93
0.92
0.91
0.90
Heavy rain
>0.25 in./h
0.89
0.88
0.86
0.84
0.82
Light snow
>0.00-0.05 in./h
0.97
0.96
0.96
0.95
0.95
Light-medium snow
>0.05-0.10 in./h
0.95
0.94
0.92
0.90
0.88
Medium-heavy snow
>0.10-0.50 in./h
0.93
0.91
0.90
0.88
0.87
Heavy snow
>0.50 in./h
0.80
0.78
0.76
0.74
0.72
Severe cold
<-4°F
0.93
0.92
0.92
0.91
0.90
Low visibility
0.50-0.99 mi
0.90
0.90
0.90
0.90
0.90
Very low visibility
0.25-0.49 mi
0.88
0.88
0.88
0.88
0.88
Minimal visibility
<0.25 mi
0.90
0.90
0.90
0.90
0.90
Non-severe weather
All conditions not listed above
1.00
1.00
1.00
1.00
1.00
Note: Speeds given in column heads are free-flow speeds.
Source: HCM 7th Edition
Table 15-5: Default SAFs by Weather Condition
Weather Type
Weather Event Definition
Capacity Adjustment Factors
55 mi/h
60 mi/h
65 mi/h
70 mi/h
75 mi/h
Medium rain
>0.10-0.25 in./h
0.94
0.93
0.92
0.91
0.90
Heavy rain
>0.25 in./h
0.89
0.88
0.86
0.84
0.82
Light snow
>0.00-0.05 in./h
0.97
0.96
0.96
0.95
0.95
Light-medium snow
>0.05-0.10 in./h
0.95
0.94
0.92
0.90
0.88
Medium-heavy snow
>0.10-0.50 in./h
0.93
0.91
0.90
0.88
0.87
Heavy snow
>0.50 in./h
0.80
0.78
0.76
0.74
0.72
Severe cold
<-4°F
0.93
0.92
0.92
0.91
0.90
Low visibility
0.50-0.99 mi
0.90
0.90
0.90
0.90
0.90
Very low visibility
0.25-0.49 mi
0.88
0.88
0.88
0.88
0.88
Minimal visibility
<0.25 mi
0.90
0.90
0.90
0.90
0.90
Non-severe weather
All conditions not listed above
1.00
1.00
1.00
1.00
1.00
Note: Speeds given in column heads are free-flow speeds.
Source: HCM 7th Edition