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What methods are used to calculate traffic speeds to make roads safer?

Calculating traffic speeds to make roads safer, less congested

Transportation safety professionals and regulatory agencies study how fast traffic is moving so they can better monitor the efficiency of roadways and relate travel speeds to trends in vehicle crashes. This information helps them to develop and oversee programs that promote safe and efficient travel for all types of vehicle.

Westat collected nationally representative traffic speeds for the National Highway Traffic Safety Administration (NHTSA). We provided speed estimates for all vehicle types on different types of roads across all U.S. geographic regions.

  • We first classified the roads as limited access highways, arterials, and collectors using sources such as Census TIGER data.
  • We selected a nationally representative sample of sites across the U.S.
  • We trained technicians to use different devices depending on what roads they were monitoring to collect speed and vehicle type information over 24 hours.
  • For major highways, we collected data using trailers with smart sensor radar units. Speed data for artrials and collectors were recorded using HiStars installed in the center of travel lanes.
  • At the end of each 24-hour period, data were uploaded to computers and sent to Westat.
  • We coordinated our efforts with the local police, who shut down roads briefly to allow our technicians to safely install and retrieve their equipment.

We collected data on more than 12 million vehicles on 677 roads.

We compared our results to similar surveys we conducted in 2009.

We delivered to NHTSA detailed information on average travel speeds for passenger vehicles, large trucks and buses, and motorcycles for major road types in U.S. regions.

Federal and state transportation agencies will use the survey data to

  • Identify travel behaviors
  • Define congestion and excessive speed-related safety problems
  • Evaluate the effectiveness of speed reduction efforts
  • Develop new approaches to promote safe driving

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