WSDOT uses the I-5 and I-90 express lanes to improve the flow of traffic during busy travel times. These lanes are opened to traffic in the direction that will relieve the most congestion from the transportation system. The express lanes are operated on a daily schedule that changes on the weekends and occasionally for major events.
Monday through Thursday, the I-90 express lanes run westbound from 6 a.m. to 12:30 p.m. and eastbound from 2 p.m. to 12:30 a.m. The express lanes are closed from 12:30 to 2 p.m. and 5 a.m. to 6 a.m. so crews can safely reverse the direction of the express lanes.
This analysis is part of a periodic review to determine the best direction for the express lanes on weekdays when special events, such as Mariners games, are scheduled at 7:10 PM.
Average Game Day: April - June 2010
The dates considered in this analysis are 7:10 PM game days on Tuesdays, Wednesdays and Thursdays in April, May and June 2010. Below is the Mariners schedule from April to June 2010 where the shaded dates represent home games.
Figure 1: Mariners Schedule
Figure 2, shown below, is a chart showing the total volume of traffic on I-90 at the West Highrise of the floating bridge. Eastbound traffic peaks at approximately 5:15 PM with more than 7,000 vehicles per hour (vph) and begins to decline soon after. The express lanes allow more vehicles per hour, approximately 1,500 vph more, to travel eastbound compared to westbound. Figure 2 shows that westbound I-90 has a capacity of approximately 5,200 vph, and stays at capacity from 4 to 6:30 PM.
Figure 2: Game Day Volumes on I-90
Average Non-Game Day
The average non-game day follows a similar profile. Eastbound traffic peaks at approximately 5:15 PM with 7,000 vph and begins to decline soon after. Westbound traffic stays at a capacity volume of 5,200 vph from 4 to 6 PM.
Figure 3: Non-Game Day Volumes on I-90
Heavy congestion builds when westbound I-90 reaches its capacity of approximately 5,200 vph; on game days, heavy congestion exists from 4 to 6:30 PM, which is 30 minutes longer than non-game days. Why not run the express lanes westbound during that time? It typically takes around an hour for an operator to switch the direction of the express lanes, including driving through the express lanes to switch gates and signs at ramps. The express lanes must be closed to assure driver and worker safety while the switchover occurs. Furthermore, any stalled vehicles in the express lanes must be removed before the express lanes can be switched, which can increase the closure duration. Closing the express lanes during rush hour to switch from eastbound to westbound would increase traffic congestion. Without the express lanes in the evening, eastbound traffic would exceed its capacity rate of 5,200 vph, and congestion would build from Rainier Ave to I-5. Southbound I-5 would be affected by eastbound I-90 congestion because approximately 40 percent of eastbound traffic crossing the bridge comes from southbound I-5. Ultimately, eastbound traffic exceeds westbound during the 2 to 7:30 PM commute, as shown in figure 4, which is why the express lanes run eastbound during the PM commute. Figure 4: Average Game Day vs Non-Game Day, Total Volumes East and Westbound, 2 to 7:30 PM
Games at 7:10 PM increase traffic and congestion westbound; however, the volume of eastbound traffic leaving Seattle is greater during the evening commute. Running the express lanes westbound would have negative effects on traffic in Seattle by increasing congestion for eastbound traffic. Further analysis is required to quantify the potential impact on I-5 and on the City of Seattle. Presently, the current express lane configuration will relieve the most congestion from the transportation system during non-game and game days.
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