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

Appendix C: Speed/Capacity for Incidents

There are two comprehensive sources of information on the effects of incidents on freeway free-flow speeds and capacities: The 2010 Highway Capacity Manual, and the SHRP 2-L08 Final Report. The HCM 2010 capacity adjustments for incidents (in terms of the proportion of original freeway capacity remaining while the incident is present) are given in Table 40. The SHRP 2-L08 project converted the HCM 2010 adjustments into adjustments to be applied to the remaining lanes open during the incident. In addition the adjustments were extrapolated to four-lanes blocked. The SHRP 2-L08 capacity adjustments are shown in Table 41.

Table 40: Residual Freeway Capacity in Incident Zones per the HCM
Number of Lanes (One Direction) Before Incident Shoulder Disablement Shoulder Accident One Lane Blocked Two Lanes Blocked Three Lanes Blocked
2 0.95 0.81 0.35 0 N/A
3 0.99 0.83 0.49 0.17 0
4 0.99 0.85 0.58 0.25 0.13
5 0.99 0.87 0.65 0.4 0.2
6 0.99 0.89 0.71 0.5 0.26
7 0.99 0.91 0.75 0.57 0.36
8 0.99 0.93 0.78 0.63 0.41

Note: Entries are proportion of original freeway capacity. Source: Exhibit 10-17, 2010 HCM

Table 41: Capacity Adjustment Factors for Incident Zones per SHRP 2-L08
Number of Lanes(One Direction)Before Incident No Incident Shoulder Closed One Lane Blocked Two Lanes Blocked Three Lanes Blocked Four Lanes Blocked
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

Note: Entries are capacity adjustments to lanes remaining open during incident. N/A = Not Applicable.
Source: Exhibit 36-16, Draft Chapter for HCM, SHRP 2-L08

Freeway Free-Flow Speed Adjustments for Incidents

Neither source (HCM nor SHRP 2-L08) identifies free-flow speed effects of incidents. The analyst might consider a free-flow speed reduction in the incident section to account for “rubber necking” by drivers distracted by the incident.

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