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Defining Congestion: Waits and Measures

Congestion is easy enough to recognize when you’re sitting in traffic. But transportation officials don’t follow a uniform standard in determining whether a road is adequately serving the need.

Waiting through two stoplights at a rural intersection might be unacceptable while covering a few miles of the 101 freeway in Los Angeles in only half an hour is a city driver’s dream. Transportation officials may even find a degree of congestion is acceptable in that it encourages drivers to look for alternatives, such as carpooling on reserved lanes or using public transit.

Researchers can measure the cost of delays, and transportation officials can make decisions to prevent them. But deciding when an acceptable delay turns into an unacceptable one is an inexact science.

The Federal Highway Administration has three congestion tests. Hours of congestion measures the amount of time freeways are moving at less than 50 miles an hour. The Travel Time Index shows the time lost on a trip on an average day. An index of 1.30 means a 20-minute trip in free-flowing traffic takes 26 minutes. The Planning Time Index shows the penalty to be on time at least 95 percent of the time, or late for work once a month. A PTI of 1.60 indicates a trip that could take 20 minutes in free-flowing traffic takes more than 32 minutes only once a month.

In the 2012 highway reauthorization known as MAP-21, Congress offered some of its own ideas about acceptable road performance and required a remedy for shortcomings. Lawmakers said the minimum average speed in high-occupancy vehicle lanes should be 45 miles per hour for any lane with a speed limit above 50 miles per hour. A high-occupancy lane that isn’t achieving that 90 percent of the time in rush hour over 180 consecutive days has a problem.

As another requirement of MAP-21, Federal Highway Administration is setting in motion a process to establish rules related to congestion and road system performance that will require states to establish targets related to the issue, according to Neil Gaffney, an FHWA spokesman.

This piece originally appeared as part of the cover story of the April 20 CQ Weekly. 



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