NCPA - National Center for Policy Analysis


May 17, 2010

Seattle Mayor Mike McGinn has proposed to replace the high occupancy vehicle (HOV) lanes planned for the new State Route (SR) 520 bridge with a transit-only configuration, like light rail.  The mayor even commissioned a study to see what it would take to make the new bridge light rail compatible.  Missing from this debate is the negative impact eliminating the HOV lanes would have on traffic.  The City's study generally concludes that traffic efficiency would be degraded, but does not really say by how much, says the Washington Policy Center.

For example:

  • The current structure only has four lanes, which according to the Washington State Department of Transportation (WSDOT) accommodates about 115,000 cars per day.
  • The Supplemental Draft Environmental Impact Statement (SDEIS) on SR 520 shows the preferred six lane option, which would add two HOV lanes (one in each direction), would carry about 131,000 cars by 2030 (20,100 of these cars would be HOVs).

By eliminating the HOV lanes, traffic volumes would increase in the remaining four lanes by 18 percent, for a total of about 131,000 cars per day.  This is about the same number of cars (135,000) that would be in the same four lanes under the no-build option found in the SDEIS.  Under this scenario, the SDEIS estimates the following will happen:

  • Daily traffic demand across Lake Washington would increase by 17 percent on SR 520, 34 percent on I-90, and 29 percent on SR 522.
  • On SR 520, morning peak period demand would increase 10 percent and afternoon peak period demand would increase 16 percent compared to today; peak period congestion would be worse than today.
  • By the year 2030, I-405 congestion will back up onto SR 520 and have substantial effects on travel times between I-5 and SR 202.

Source:  Michael Ennis, "520 traffic congestion would grow 10 percent to 16 percent under McGinn's plan," Washington Policy Center, April 9, 2010.

For study:


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