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

Appendix F: Speed/Capacity for Incident Duration Reductions

Reductions in incident duration due to TIM strategies are estimated by the analyst for each incident type. Incident duration is the sum of the detection, verification, response, and clearance times for the incident. A value of 1.00 for the incident duration factor means no change to the incident duration from the “before-ATDM” condition. A value of 0.90 means a 10% (1-0.90) reduction in the incident duration. Since the smallest temporal unit employed in the 2010 Highway Capacity Manual freeway analysis method coded in FREEVAL-ATDM is 15 minutes, the effects of small reductions in incident duration are approximated by increasing the 15-minute capacity of the freeway using the formulae below.

Figure 10: Capacity Gained by Reducing Incident Duration

Figure 10 is a graph showing Capacity (vehicles per hour) over Time (hours). Capacity is up at Non-incident capacity until the start of incident, where it drops to incident capacity. The original incident duration is shown, followed by a return to Non-incident capacity. Also shown is the reduced incident duration due to traffic incident management, and the capacity gained by the reduction of incident duration.

Source: 2010 Highway Capacity Manual.

As shown in Figure 10, the capacity gained by shortening the incident duration is:

Equation to determine the capacity gained by shortening the incident duration.

Where:

CapGained = capacity gained (vehicles)
C1 = capacity prior to and after incident (vph)
C2 = capacity during incident (vph)
T = duration of incident (hours)
X = proportional reduction in incident duration (unit less)

The new average capacity (caused by reduction of incident duration, but measured over the entire original period of the incident) is:

Equation for the new average capacity.

Where:

AveCap = average capacity over original incident duration (vph).

All other variables are same as before.

The original capacity adjustment factor for the incident (y = c2/c1) becomes AveCap/c1

Equation to determine the original capacity adjustment factor for the incident.

Where:

AveCapFac = new average capacity adjustment factor reflecting shortened incident duration (unit less)
y = original capacity adjustment factor for incident (unit less)
x = proportional reduction in incident duration (unit less)

A similar approach is used to identify the new average speed adjustment factor of incident with shortened duration.

Equation to determine new average capacity adjustment factor reflecting shortened incident duration (unit less).

Where:

AveSpdFac = new average speed adjustment factor reflecting shortened incident duration (unit less)
z = original free-flow speed adjustment factor for incident (unit less)
x = proportional reduction in incident duration (unit less)

Demand is not adjusted for the shorter incident duration.

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