I-90 Traffic Analysis
Traffic slows down on I-90 when the flow volumes reach 5,200 vehicles per hour or more, as shown in Figure 1 below. During the study period, traffic began to slow down at about 3:30 p.m., the start of evening rush hour. Traffic congestion and back-ups persisted until about 6:30 p.m., when the traffic volumes returned to fewer than 5,200 vehicles per hour.
On I-90 on a Mariners game day 17,900 vehicles travel east and 15,800 travel west between 3:30 p.m. and 6:30 p.m. This is a difference of 2,100 vehicles. Since Mariner's games started at 7:05 p.m. during the study period, we also reviewed data for a slightly extended evening rush hour. On I-90 on a Mariners game day, 32,850 vehicles travel east and 29,150-travel west between 3:30 p.m. and midnight. With more vehicles traveling east than west, the express lanes should remain open to motorists traveling east during evening rush hour, even on Mariners game days.
Figure 1: Free flow is approximately 5,200 vehicles per hour
The illusion that there's less traffic eastbound results from the fact that westbound traffic uses only 3 lanes, while eastbound traffic uses 5 lanes, including both mainline and express lanes.
After 6:30 p.m. on Mariners game days westbound traffic volumes exceed eastbound traffic volumes. Why not run the express lanes westbound during that time? It typically takes around an hour for an operator to drive through the express lanes, switching gates and signing at ramps. The express lanes must be closed to assure driver and worker safety while the changeover occurs. Closing the express lanes during rush hour to switch from eastbound to westbound would increase traffic congestion in both directions. Furthermore, any stalled vehicles in the express lanes must be removed before the express lanes can be switched, which could potentially increase the closure duration greatly.
I-90 and SR 520 Traffic Analysis
Figure 2 below takes the analysis a step further and looks at total directional volumes of I-90 and SR-520. The games that occurred on August 22-24, 2006 had a start time of 7:05 p.m. The start of the evening rush hour was at 3:00 p.m. and by 6:30 p.m., the end of the evening rush hour, approximately 31,300 vehicles have traveled eastbound and 31,050 vehicles have traveled westbound, creating a difference of 250 vehicles.
Figure 2: Free flow is approximately 8,200 vehicles per hour
For comparison, Figures 3 and 4 below provide similar graphs for non-game days. They show a wider difference between the eastbound and westbound volumes during the evening commute. Figure 3 below looks at I-90 during non-game days on August 15-17, 2006. The start of the rush hour is 3:30 p.m. with approximately 5,400 vehicles per hour traveling eastbound and 5,150 vehicles per hour traveling westbound. By 6:30 p.m., the end of the evening rush hour, approximately 18,450 vehicles have traveled eastbound and 15,750 vehicles have traveled westbound, creating a difference of 2,700 vehicles.
Figure 3: Free flow is approximately 5,200 vehicles per hour
Figure 4 below looks at I-90 and SR-520 during non-game days from August 15-17, 2006. The start of the rush hour was 3:00 p.m. and by 6:30 p.m., the end of the evening rush hour, approximately 31,800 vehicles have traveled eastbound and 30,950 vehicles have traveled westbound, creating a difference of 850 vehicles.
Figure 4: Free flow is approximately 8,200 vehicles per hour
This analysis shows that during the evening rush hours, 3:30 to 6:30 p.m., I-90 eastbound is still the larger movement on days with and without evening Mariner games. This difference is over 2,000 vehicles in both cases. If the analysis period is extended, because of the 7:05 start time of the games, to 3:30 p.m. to midnight, I-90 eastbound is again the larger movement in both cases with a difference of over 3,000 vehicles.
However, this analysis also shows that the difference between total eastbound and westbound movement and I-90 and SR-520 during the evening rush, 3:00 to 6:30 p.m. is very small – less than 1,000 vehicles on days with and without evening Mariner games. An extended analysis period, 3:00 p.m. to midnight revealed that westbound movement was larger than eastbound movement on days with and without Mariner games by over 500 and 1,000 vehicles respectively. We will pay close attention to these new trends and respond accordingly by always striving to open the express lanes on I-90 to the direction with the largest total volumes.
- I-90 Eastbound is still the larger movement in the p.m. peak period.
- We will continue to evaluate traffic conditions as they change through the years.
- If you have any questions about this report or the express lanes operations, please email us at nwflow@wsdot.wa.gov.