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Congestion and System Performance
Demand for highway travel by Americans continues to grow as the population increases, particularly in metropolitan areas. Construction of new highway capacity to accommodate this growth in travel has not kept pace. Between 1980 and 1999, lane-miles of highways increased 1.5 percent while vehicle miles of travel increased 76 percent. State DOTs examine two types of congestion delay: 1) recurring congestion (due to excess demand such as rush hour traffic and bottlenecks caused by the highway design), and 2) non-recurring congestion (generated by temporary conditions such as traffic incidents, work zones, extreme weather, and special events). It is estimated that non-recurrent congestion causes 30 percent to 60 percent of the total delay on highways.
Travel Time predictability and reliability are some of the most important performance expectations that freight movers and commuters share. As a result, many DOTs have explored ways to measure reliability and “real” travel times. Many have deployed key operational strategies such as incident response programs to reduce non-recurring congestion and increase safety and travel time reliability and maximize throughput on the existing system.
Florida’s Incident Response Program
The primary goals of the FDOT Traffic Incident Management (TIM) Program are to increase mobility and reduce secondary incidents. Florida uses a program to patrol congested areas and high incident locations along urban highways. The mission is to provide highway assistance services during incidents to reduce delay and improve safety for the motoring public and responders. In 2005, 298,776 service assists were performed, more than double the 112,000 assists performed in 2000, the first year of the program. Over 1.5 million service assists have been performed since 2000, with the busiest year of 342,895 assists in 2004.
Maryland’s Incident Response Program
The Coordinated Highways Action Response Team (CHART) is a comprehensive traffic management system and incident management program which includes emergency traffic patrols to provide emergency motorist assistance, emergency response units for overall traffic control at accident locations, and pre-stocked trailers with traffic control tools used to quickly set up pre-planned detour routes when incidents require full roadway closure.
The CHART incident management program provides safety and economic benefits to motorists and commerce in Maryland. By June 2008, this program is anticipated to save motorists and commercial traffic approximately 30 million vehicle-hours annually, equivalent to $570 million a year in cost savings through incident clearance. (Table 1.)
Table 1. Vehicle-Hours Saved Annually from 2003 to Target Date (June 2008) Maryland
Missouri’s Uninterrupted Traffic Flow
This measure is used to determine the trends in incident clearance on the state highway system. Motorist Assist and the traffic operations centers track the time for all lanes to be cleared following an incident.
This tracking approach has continued to reduce incident clearance times. Increased efforts in incident management, Motorist Assist and police coordination in both the St. Louis and Kansas City regions continue to support MoDOT’s objective of quick clearance and open roadways with the ultimate goal of improving clearance times. (Figure 6.)
Figure 6. Average Time to Clear Traffic Incident in Kansas City, Missouri, 2005–2007
Photo courtesy of Missouri Department of Transportation. MoDOT’s Motorist Assist responds to incident.
Minnesota’s Performance Measures for Measuring Recurrent and Non-Recurrent Delay
Minnesota is another state that uses real-time performance information to communicate freeway congestion and travel time data to the public. In addition, MnDOT measures the average clearance time for freeway incidents. In 2006, the MnDOT stabilized the average time for clearing incidents on Metro freeways at 38.8 minutes. The target is 35 minutes. (Figure 7.)
Figure 7. Average Clearance Time for Minnesota Urban Freeway Incidents in Metro Area in Minutes (3-Year Average)
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Increasing traffic volumes and accidents tend to continue to push clearance time upward, but strategies such as close coordination with the State Patrol and expansion of FIRST service patrol are being used to keep clearance times from increasing. |
Washington’s Performance Measures for Congestion and System Performance
Washington collects real travel time data to produce a comprehensive, annual congestion report that analyzes system performance. Key metrics include average peak travel time, 95 percent reliable travel time, vehicle throughput, lost throughput productivity, delay, percent of days that the speed falls below 35 mph, duration of congestion, incident response clearance time, and Before-and-After analysis of performance for selected highway improvement projects. An example of how Washington has been able to manage congestion with performance measures is its effort to enhance incident response, which has resulted in average clearance time of less than 20 minutes since 2003. Table 2 is another measurement example for specific travel time and reliability measures that give commuters information and provide WSDOT the needed data to make informed decisions about capital and operational congestion strategies and investments.
Table 2. Travel Time Performance for Selected Central Puget Sound Commutes, 2005*
