Trying to Get it Right on Traffic

Remember our description of Montgomery County’s screwed-up system for measuring traffic congestion? Remember our proposal for accurately measuring congestion through massive usage of GPS devices? Well, it turns out that the country’s largest traffic measurement company agrees with us.

INRIX is a traffic measurement firm based outside Seattle that was founded by two former Microsoft executives in 2004. The company just released a report ranking traffic congestion all over the United States. (The Washington area ranks fourth-worst after Los Angeles, New York and Chicago.) How does the company gather its data? The company states:

The raw data comes from the historical traffic data warehouse of the INRIX Smart Dust Network. Since 2006, INRIX has acquired billions of discrete “GPS-enabled probe vehicle” reports from commercial fleet vehicles – including taxis, airport shuttles, service delivery vans, long haul trucks – and cellular probe data. Each data report from these GPS-equipped vehicles includes at minimum the speed, location and heading of a particular vehicle at a reported date and time.

INRIX has developed efficient methods for interpreting probe vehicle reports that are provided in real-time to establish a current estimate of travel patterns in all major cities in the United States. These same methods can aggregate data over periods of time (annually in this report) to provide reliable information on speeds and congestion levels for segments of roads. With the nation’s largest probe vehicle network, INRIX has the ability to generate the most comprehensive congestion analysis to date, covering the nation’s largest 100 metropolitan areas.

How do they measure congestion? The company calculates a “reference speed” based on how fast GPS-equipped vehicles travel on a road in the middle of the night. Presumably, that reflects driving time in non-congested conditions. Then the company draws on more GPS data to calculate average speeds in each hour of the day for every day of the week. Then the company divides the reference speed (representing free flow) by the average peak-hour driving speed to calculate a Travel Time Index. The higher the index, the greater the congestion. For example, an index value of 1.3 indicates that a peak-hour trip will take 30% longer than a free-flow trip because of congestion.

Is INRIX’s congestion formula the right one? Maybe yes, maybe no. But more importantly, their calculations are based on billions of actual trips recorded by GPS devices in commercial vehicles all over the U.S. Unlike Montgomery County, INRIX does not base its statistics on fluky critical lane volume measurements that are taken once every four years or so and, according to Park and Planning’s own research, do not actually measure congestion.

Folks, we have to be able to measure traffic congestion accurately in order to plan successful mitigations, including road improvements and transit. Here’s a private sector company that is getting a ton of real-world data and giving it their best shot. So if INRIX is doing it, why can’t we?