Traffic counts will inform school planning near high school block

Driving through major Island intersections during peak commute times, or when a special event is scheduled, traffic is sure to be heavy. While drivers know that in theory, now there is data to back it up.

On Dec. 3, the Mercer Island School District Board of Directors received a report on traffic volumes done by a Seattle transportation consultant, Transpo, which looked at traffic near and around the district’s North Mercer Campus.

Traffic counts were collected in October 2012 from Oct. 17 through Oct. 25, looking at the hourly traffic patterns on different days of the week, and included Friday, Oct. 19, the night of the homecoming football game at the high school.

Transpo also collected data at Island Crest Way and S.E. 40th Street; Island Crest Way, Merrimount Drive and S.E. 44th Street, as well as East Mercer Way and S.E. 36th Street.

Traffic along S.E. 40th Street during a normal weekday is fairly standard, with peaks during the morning and evening commute times.

During school day afternoons, however, there are two peak periods.

The data shows that evening commute peak hour volumes on S.E. 40th Street are approximately 100 to 200 vehicles per hour higher than those occurring during school afternoon peak hour.

Yet, on Oct. 17, the same day as a Seattle Sounders game in downtown Seattle, showed a spike in cars along S.E. 40th Street, adding 200 vph more from 6 to 7 p.m. than the average from the same time earlier in the week.

Since 2005, the last time a traffic study was done, tolls along State Route 520 have been put into place, affecting traffic along some Island streets and intersections.

In the mornings, the largest increase was at the intersection of S.E. 40th and 86th Avenue S.E. with 14 percent more cars, while Island Crest Way and Merrimount dropped 8 percent.

The afternoon commute changed the most. There was a 48 percent increase at 86th Avenue and S.E. 42nd Street, where just 315 cars traveled in 2005, jumping to 465 cars in 2012. The intersection of 92nd Avenue and S.E. 42nd Street also jumped, increasing 39 percent, while both the intersections at S.E. 40th Street and 92nd Avenue and S.E. 40th and 86th Avenue increased by 25 percent.

However the addition of new schools in the block will not have a similar impact.

For example, a new K-5 elementary school on the North Mercer Campus would only cause “minimal change to the current existing trip generation.” While a new school would displace some parking for the high school, it would not require additional driving lanes.

With a 650 student middle school, the morning peak hour would likely include 351 trips with 193 coming and 158 leaving, while the study found around 202 trips are likely for the school afternoon peak.

With this data, the district will conduct another phone survey this month.

To see the report, visit the MISD website at

How many walk, drive to MIHS

Since 2010, MISD has given a survey to students at Mercer Island High School asking them how they get to school. The report is required as part of the PEAK agreement.

According to the report, 1,261 students were surveyed, equaling 86 percent of the school population.

Students answered if they carpooled to school, drove alone, took the bus, walked or biked.

• Fifty percent (636 students) said they carpool to school; 20 percent (246 students) said they drove alone.

• Eighteen percent (225 students) took the bus.

• 11 percent (136 students) walked and 1 percent (18 students) biked.

Compared to 2011, the number of students in each category increased except walking, which went down from 13.1 percent in 2011 to 10.8 in 2012.

Of the students who live in the walk zone, 178 said they walked or biked, while 135 drove. In 2011, 209 walked and biked while 120 drove.


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