I-90 Speed Limit Study

Result of Study

After careful review, analysis and public input, the Washington State Department of Transportation, Washington State Patrol and Washington Traffic Safety Commission announced May 11, 2016, that, due to safety concerns, they will not increase speed limit on Interstate 90 in eastern Washington.

The area under consideration about a 100-mile stretch of Interstate 90 that runs roughly from George to the west Spokane County line.

WSDOT worked with the state Traffic Safety Commission and the Washington State Patrol to develop criteria to use in considering raising speed limits from 70 to 75 mph. The process included:

  • gathering information from other states that have raised speed limits
  • developing criteria to evaluate where and how speed limits might be raised
  • assessing the effects of an increase and identifying any needed highway feature modifications
  • examining how speed limit increases can affect crashes, societal costs due to crashes, fuel costs and benefits associated with travel time savings.

Why did WSDOT study a speed limit increase?

  • A bill passed in the 2015 Legislature (RCW 46.61.410) allows the Secretary of Transportation to increase the speed limit on any highway to a maximum 75 mph.
  • Lawmakers in eastern Washington have asked WSDOT to consider such an increase on I-90.
  • If an increase was approved, the limit would remain 60 mph for vehicles over 10,000 pounds or vehicles pulling trailers.

The end result
This section of I-90 is posted at 70 mph and the average speed (the speed that half the drivers are traveling at) is 73 mph. Findings from other states that have increased speed limits above 70 mph found that the speeds increase between 1 and 4 mph. If drivers in Washington follow the trend found in other states, average passenger vehicle speeds on I-90 would likely increase up to 4 mph.

We estimated potential changes in travel times, crashes, fuel consumption and greenhouse gas emissions at both 2 and 4 mph increases in average passenger vehicle speeds.

Potential Travel Time Changes

  • Average passenger vehicle travel time from George to near the west Spokane County line is currently 90.4 minutes.
  • With a 2 mph speed increase, the average trip decreases by 2.4 minutes to 88 minutes. That's an estimated savings – in drivers' time – of $1.86 million a year.
  • With a 4 mph speed increase, average travel time decreases by 4.7 minutes, for 85.7 minutes. Estimated value of time saved is $3.6 million annually.

Potential Crash Changes

  • The five year crash history (starting in 2011) shows a total of 1,079 crashes on this stretch of roadway, with 15 fatal and 28 serious injury collisions. Using national research and societal cost numbers, we can expect to see the following if the speed limit was increased:
  • At a 2 mph average speed increase: our review of data suggests an additional 0.62 fatal/serious injury crash annual and an additional societal cost due to crashes of about $4 million.
  • At a 4 mph average speed increase: our review of data suggests an increase of 1.27 fatal/serious injury crashes annually and an additional societal cost of $8.3 million

Potential Fuel Consumption Changes

  • Existing fuel consumption for the trip is estimated to be 20.7 million gallons a year.
  • 2 mph increase: an additional 735,000 gallons per year at a cost of about $1.6 million.
  • 4 mph increase: an additional 1.5 million gallons per year at a cost of $3.5 million.

Estimated Greenhouse Gas Changes

  • Carbon dioxide equivalent per year
  • Current: 135,700 metric tons per year
  • 2 mph increase: an additional 2,245 metric tons per year, at an additional cost of $132,000.
  • 4 mph increase: an additional 4,490 metric tons per year, at an additional cost of about $265,000.

What is the project timeline?

  • The decision was released May 11, 2016