Guidelines for Quantifying Benefits of Traffic Incident Management Strategies (2022)

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52 Reduction in Available Capacity from Incidents The 2010 HCM serves as a worldwide reference for transportation and traffic engineering scholars and practitioners. Within the HCM are tables defining available freeway segment capacity when a number of freeway and shoulder lanes are blocked by incidents. The HCM table parameters, defined as the available capacity ratio (ACR), represent the effective capacity of a facility during lane blockage. The ACR is one of the three key variables in estimating delay from an incident. The other two key variables are demand and incident duration. Recent improvements in the detail and quantity of data collected to describe incidents enhance the ability to review these estimates and update findings to include more varied road conditions. Two general approaches have been applied to estimate ACR either through video data analysis or field detector volume and speed data analysis. Prior studies focus on two- and three-lane highways, while the present study extends to five-lane facilities. The prior studies did not use the same flow-based parameters to measure capacity before and during incidents, which likely explains some of the variations in the results. With the exception of one study, all of the studies used data from a single jurisdiction. The present study considers three separate jurisdictions, enabling consideration of the effects of regional factors. Analysis of empirical data from 109 incidents on freeway facilities in Maryland, Virginia, and Texas indicates that the HCM values underestimate the effect of shoulder incidents on ACR, while overestimating the effects of incidents that block main lanes. Further, the differentia- tion HCM presents for shoulder disablements and shoulder accidents may not be warranted. The analyses in this study suggest the capacity loss from shoulder incidents and accidents is comparable. Finally, analyses confirm that capacity loss will vary significantly from region to region. For example, the capacity loss for comparable incidents is lesser on Maryland facilities and greater on Texas facilities, compared to Virginia. This analysis comprises incidents that occur during peak traffic and good visibility. The cumulative counts slope analysis method was applied to accurately estimate the traffic capacity under both normal and incident conditions. Incidents were selected based on the following criteria: • Incident location in the vicinity (<0.5 mile) of upstream and downstream detectors. • No access points between the upstream and downstream detectors. • Active bottleneck conditions (demand surpassing capacity) during the incident. In Table C1 are the current HCM estimates of ACR (bold black), along with a summary of this (italic blue) and six other studies on capacity reduction due to incidents. In conducting analyses of the delay effects from quick incident clearance, the TIM agency should confirm whether broader agency-wide ACR tables are prescribed. If they are not, then the agency should rely on HCM parameters or consider the capacity loss parameter sets that better reflect their region. A P P E N D I X C

Reduction in Available Capacity from Incidents 53   Proportion of Freeway Segment Capacity Available Under Incident Conditions Number of Lanes (One Direction) Shoulder Disablement Shoulder Accident One Lane Blocked Two Lanes Blocked Three Lanes Blocked 2 0.9511 0.8110 0.68–0.7712 0.3510 0.46–0.5011 0.47–0.5013 0.0010 0.13–0.1411 N/A 3 0.9910 0.5214 0.79 0.8310 0.7413 0.7215 0.77 0.4910 0.40–0.4312 0.5013 0.3614 0.3716 0.55 0.1710 0.29–0.3212 0.2113 0.1814 0.2315 0.24 0.0010 4 0.99 10 0.91 0.8510 0.87 0.5810 0.69 0.2510 0.30 0.1310 0.18 5 0.99 10 0.8710 0.6510 0.76 0.4010 0.60 0.2010 0.32 6 0.9910 0.8910 0.7110 0.5010 0.2610 7 0.9910 0.9110 0.7510 0.5710 0.3610 8 0.9910 0.9310 0.7810 0.6310 0.4110 Note: Bold text indicates HCM values, italic indicates estimates from this study. Table C1. Comparison of HCM estimates of ACR with this study and other studies on capacity reduction due to incidents. 11 Highway Capacity Manual, Transportation Research Board, Washington, D.C., 2010. 12 Lu, C., and L. Elefteriadou. An Investigation of Freeway Capacity Before and During Incidents. Transportation Letters: the International Journal of Transportation Research 5(3), pp. 144–153, 2013. 13 Knoop, V. L., S, P. Hoogendoorn, and H. J. van Zuylen. Capacity Reduction at Incidents: Empirical Data Collected from a Helicopter. Transportation Research Record, No. 2071. Transportation Research Board, Washington, D.C., pp. 19–25, 2008. 14 Goolsby, M. Influence of Incidents on Freeway Quality of Service. Highway Research Record, No. 349. Highway Research Board, Washington, D.C., pp. 41–46, 1971. 15 Knoop, V., S. Hoogendoorn, and K. Adams. Capacity Reduction at Incidents Sites on Motorways. European Journal of Transport and Infrastructure Research, Vol. 9, No. 4, pp. 363–379, Dec. 2009. 16 Smith, B. L., L. Qin, and R. Venkatanarayana. Characterization of Freeway Capacity Reduction Resulting from Traffic Accidents. Journal of Transportation Engineering, 129(4), pp. 362–368, 2003.