Mercer Island has over 75 miles of public roads. Interstate 90 (I-90) runs east-west across the northern end of Mercer Island, providing the only road and transit connection to the rest of the Puget Sound region. Access to the I-90 on-ramps and off-ramps is provided at West Mercer Way, 76th Avenue SE, 77th Avenue SE, 80th Avenue SE, Island Crest Way, and East Mercer Way.
Most of the road network is comprised of 2-lane local streets serving the Island’s residential areas. Arterial roadways comprise approximately 25 miles, or one third, of the system. In addition to public roads, there are numerous private roads serving individual neighborhoods and developments on the Island.
Roadways on the Island are classified into different categories according to their purpose and physical characteristics. The categories are:
•Principal Arterials carry the highest volumes of traffic and provide the best mobility in the roadway network. These roads generally have higher speed limits, higher traffic volumes, and limit access to adjacent land uses.
•Secondary Arterials connect with and augment principal arterials and generally have a higher degree of access to adjacent land, lower traffic volumes and lower travel speeds.
•Collector Arterials provide for movement within neighborhoods, connecting to secondary and principal arterials; and typically have low traffic volumes and carry little through traffic.
•Local Streets provide for direct access to abutting properties and carry low volumes of traffic at low travel speeds. Local streets are usually not intended for through traffic.
Individual streets are assigned classifications based on several criteria, including the type of travel to be served, the role of the street in the overall street network and transportation system, physical characteristics, traffic characteristics, and adjacent land uses. Based on City Staff recommendations, the City Council periodically reviews and updates the street classification system, its criteria and specific street classification designations.
Figure 1 shows the street functional classifications. Figure 2 shows the number of travel lanes and Figure 3 shows the posted speed limits of arterial roadways.