Traffic Impacts of an SOV Mode Split Target Policy: Evaluating Seattle’s New Transportation Concurrency System

Abstract

Cities in Washington State are required to adopt concurrency policies that stipulate how they will measure the level of service (LOS) of their transportation system. Under the Growth Management Act, cities are also required to deny development permits if they do not meet level of service standards they set. In an effort to refocus their transportation system on moving people, Seattle recently adopted a concurrency system that measure LOS by single occupant vehicle (SOV) mode split but exempts the densest areas of the city. Studies by the city concluded that there will be no impacts to traffic under the new policy. This research scrutinizes that assumption by examining trip generation along selected corridors under both current development and future development scenarios using the output of the city’s zoned capacity model. I found that SOV trips are likely to increase in exempted areas even if mode split aligns with targets set in the concurrency policy. This suggests that traffic conditions will worsen under the new concurrency policy and brings into question its efficacy as a policy to reduce impacts to the city’s transportation system.