15.4.4 Factors Affecting Travel Time Reliability

The factors that affect travel time reliability tend to adjust either the demand placed upon a facility, the effective available capacity of the facility, or the base travel speed that vehicles travel at while using the facility. Each factor is described in greater detail below.

15.4.4.1 Traffic Demand

Traffic demand fluctuates both across days within the week and across months of the year. Daily and weekly fluctuations can be caused by work schedules, typical working business hours, and other factors. Monthly fluctuations can be caused by seasonal activities, such as sporting events, recreational activities, and school session schedules. Traffic demand also fluctuates across the year due to holidays, religious festivals, fairs, concerts, and other special events.
The base freeway analysis scenario accounts for traffic demand variation for different hours of a given day. To account for day-to-day traffic demand variability in the generation of multiple scenarios, the traffic demands for the analysis period are calibrated by applying a demand multiplier based on the day of week and month combination. The demand multiplier is a ratio of the day-month combination daily facility demand to the daily facility demand for a reference period. These factors are first used to adjust demand data used in the base scenario to what may be expected in terms of demand for the reference period. A second adjustment changes the demand of the reference day-and-month combination to the expected demand for any other day-month combination analyzed as part of the RRP. and provide default demand ratios for urban and rural freeways, respectively, with ratios shown relative to traffic demand on a Monday in January. These values can be supplemented by locally collected demand factors if they are available.

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

15.4.4.3 Traffic Incidents

The base scenario assumes the freeway is operating at full capacity with no traffic incidents or closures. However, the impacts of a traffic incident on freeway operations can vary depending on the severity of the incident and the extent of the resulting closure. Incidents typically cause shoulder or lane closures, as disabled vehicles block the roadway, and vehicles passing incidents tend to drive slower, reducing the capacity of the freeway segment.
Traffic incidents are generated for this analysis based on their expected frequency of occurrence on the facility within the analysis hours of a day in a given month. The incident frequency is the average number of incidents experienced on the facility and varies each month.
For scenarios with an incident, the incident details (including number of lanes impacted and incident duration) are modeled using a normal probability distribution. provides parameters for normal distributions of the impacts an incident may have, depending on the severity of the resulting closure. These parameters are used to randomly generate and assign the characteristics of an incident to use for an analysis scenario. displays the CAFs associated with each incident severity type, and these factors are applied based on the incident with characteristics generated from the parameters shown in . For example, if an analysis segment has a 50 percent chance of experiencing an incident, then for half of the scenarios generated for a month, each scenario is modeled with a unique incident.
The incident severity and duration parameters are randomly generated from the normal distribution referenced in . Additionally, shows that a three-lane closure incident on a four-lane directional facility results in a loss of three full-lane capacities and maintaining only 52 percent of the capacity in the remaining lane. The CAFs in are applied to each scenario for that scenario’s randomly generated incident duration and according to that scenario’s randomly generated incident severity. The CAFs are applied to the capacity results to determine the impacts of incidents on roadway capacity based on the scenario being analyzed.
Table 15-6: Default Freeway Incident Severity Distribution and Duration Parameters (min)
Parameter
Incident Severity Type
Shoulder Closed
1 Lane Closed
2 Lanes Closed
3 Lanes Closed
4+ Lanes Closed
Distribution (%)
75.4
19.6
3.1
1.9
0
Duration (mean)
34
34.6
53.6
67.9
67.9
Duration (standard deviation)
15.1
13.8
13.9
21.9
21.9
Duration (minimum)
8.7
16
30.5
36
36
Duration (maximum)
58
58.2
66.9
93.3
93.3
Source: HCM 7th Edition
Table 15-7: CAFs by Incident Type and Number of Directional Lanes on the Facility
Directional Lanes
No Incident
Shoulder Closed
1 Lane Closed
2 Lanes Closed
3 Lanes Closed
4 Lanes Closed
2
1.00
0.81
0.70
N/A
N/A
N/A
3
1.00
0.83
0.74
0.51
N/A
N/A
4
1.00
0.85
0.77
0.50
0.52
N/A
5
1.00
0.87
0.81
0.67
0.50
0.50
6
1.00
0.89
0.85
0.75
0.52
0.52
7
1.00
0.91
0.88
0.80
0.63
0.63
8
1.00
0.93
0.89
0.84
0.66
0.66
Notes: N/A = not applicable – the number of lanes closed equals or exceeds the number of directional lanes. The methodology does not permit all directional lanes of a facility to be closed.
Source: HCM 7th Edition