Study Shows No Significant Impact to Mercer Island

Joe Wolf (Flickr)

Last Wednesday, CH2M submitted the I-90 and Mercer Island Mobility Study prepared on behalf of Sound Transit.

If you've not been following the drama, a quick recap: Mercer Island has been making considerable noise since 2015 about perceived loss of mobility due to East Link construction. Whereas prior complaints were more generic and white-hot, of late the complaints have been much more focused on mitigation for the closing of the express lanes for East Link construction and the coming SOV-to-HOV conversion of the westbound ramp from Island Crest Way to I-90. Islanders maintain that they are owed mitigation for this by the 1976 agreement granting their solo vehicles special access to the I-90 express lanes, and also by a 2004 amendment to the agreement holds that:

[t]o the extent of any loss of mobility to and from Mercer Island based on the outcome of studies, additional transit facilities and services such as additional bus service, parking available for Mercer Island residents, and other measures shall be identified and satisfactorily addressed by the Commission, in consultation with the affected jurisdictions."

To date, WSDOT and Sound Transit have held that the agreement clearly and permanently committed the center lanes for transit and that Mercer Island SOV access was a temporary and conditional use. The agencies noted in a letter to Mercer Island that SOV use of HOV lanes, even temporarily, would be a violation of federal law and a trigger to repay grant funds to the federal government. In response, Mercer Island announced its intent to sue Sound Transit, threatening both the schedule and budget for East Link.

Things have softened a bit since then. Mercer Island reneged on its threat to revoke shoreline permits for Sound Transit, and the ST Board committed in March that the city and agencies meet regularly to negotiate issues around Mercer Island Station beyond traffic. The current study was an olive branch between the agencies and the city, seeking data to quantify the scale of any lost mobility, in the spirit of the 2004 amendment.

Well, the study results are out. In a memo to Sound Transit board members last week, Secretary Millar summarized that:

the Mobility Study concludes that the overall mobility for people traveling to or from Mercer Island will be similar to or slightly improved compared to existing conditions during the six-year East Link construction period, and will be improved once East Link light rail service begins in 2023. A short summary of the study is attached. [emphasis mine]

The study examined five options. The No-Build Alternative would cancel East Link. Option 1 would maintain the current construction schedule and roadway plan, but allow Mercer Island SOVs to use the HOV lane, whereas Option 2 (the current plan) would not. Option 3 would cancel the HOV lane plan and all lanes of westbound I-90 would be general purpose from Island Crest Way to I-5. Option 4 would allow temporary use of the HOV lanes for SOVs using the westbound ramp, but would require an immediate merge into the general lanes. The No-Build Alternative isn't a serious threat, and WSDOT notes that only Options 2 and 3 comply with federal law.

The study notes that Island Crest Way only serves 20% of Mercer Island's SOV traffic onto westbound I-90, with just 2,000 vehicles daily. The other two primary access points, West Mercer Way and 76th Ave SE, carry 4,000 vehicles each. During peak periods, the combined peak hour volume for Island Crest Way is approximately 300 vehicles, with less than 100 vehicles typically entering via the ramp during the morning commute peak hour.

Crash risk would be increased with Option 4, as higher differential speeds between the GP lanes and the HOV lane would increase the risk of required merging. And general non-compliance would be a fatal blow for Option 1, as other area commuters would quickly learn that there would be no realistic way to enforce the HOV requirement west of Mercer Island.

A 50 percent HOV non-compliance rate was used for modeling the operation of this option in order to reflect a realistic expectation that with limited means for enforcement, many SOV motorists will choose to stay in the HOV lane traveling across the bridge. This non-compliance may also encourage other mainline travelers to do the same. The required left to right merge from a higher speed HOV lane to a congested GP lane increases risk of crash frequency.

The current plan (Option 2) scores the best in terms of speed and throughput:

Construction Option 2 is the only option that meets the HOV speed and reliability policy requirement of 45 mph or greater, at least 90 percent of the time, for travel between Bellevue and Seattle.

In terms of total travel time, Option 2 greatly improves transit and HOV speeds during the 6-year construction period, saving 2.3 minutes per trip. SOVs would see a 2 minute increase in AM westbound travel time. But the increased inefficiency and merging pressure of Option 4 (where SOVs would be allowed but required to merge) would be even worse for Mercer Island SOVs, adding an additional 24 seconds per trip compared with Option 2. Option 3 (canceling the HOV lane entirely) would save SOVs a mere 6 seconds over Option 2.

You can read the full study here, or WSDOT's Summary Memo here. But the results are clear: there is no reason to delay East Link on account of a mere 300 vehicles per AM/PM peak period. Travel times for Islanders will not be significantly worsened, and 76th Ave SE and West Mercer Way are able to take up much of the slack. The current plan is the only one that complies with federal law. And six years later, Islanders will have all-day, traffic free, high capacity transit the likes of which they've never seen.

Any mitigation that seeks to increase access to the Island Crest Way ramp should focus on increasing the HOV rate. And if budgetary resources allow, agencies should shower the island with additional peak transit service, whether in the form of more hours on Routes 204, 550, or 630, or to a resurrected Route 202. But East Link construction should proceed on schedule, and I-90's thousands of current transit commuters deserve to have complete HOV/Transit priority across Lake Washington.