December 18, 2015
Express lanes concept
The Washington State Department of Transportation uses express lanes on I-5 and I-90 to improve traffic flow during peak travel times. These lanes are opened in the direction with the highest demand. The express lanes are operated on a weekday schedule and a weekend schedule. The schedule is sometimes changed to accommodate major events, construction work, and maintenance activities.
The weekday schedule for the I-5 express lanes service is as follows:
- Southbound: 5:00 a.m. to 11:00 a.m.
- Northbound: 11:15 a.m. to 11:00 p.m.
The express lanes service is closed during the midday period from 11:00 a.m. to 11:15 a.m. so crews can safely reverse the direction of the express lanes. It is also closed overnight from 11:00 p.m. to 5:00 a.m. to reduce nighttime noise for residents near the Ship Canal Bridge.
Purpose of analysis
This study analyzes weekday traffic flow to determine whether the express lanes service operates in the direction that will best reduce traffic congestion. It also evaluates whether the express lanes service appropriately serves traffic volumes on days with special events, such as Mariners, Sounders FC, and Seahawks games, that are scheduled at approximately 7:10 p.m.
Figure 1 below shows the dates used for analysis from June 1, 2014 to November 30, 2014.
Traffic week data collected from June through November 2014 were used for this study. Dates without major sporting events have been analyzed separately; dates with major sporting events were selected if there was a game starting between 12:00 p.m. and 7:10 p.m.
While only some of the information is presented in this report, the express lanes study considered all of the following factors:
- Northbound and southbound volumes
- Northbound and southbound delay
- Overall congestion
- Access to and from the express lanes
- Demand for destinations serviced by the express lanes, and
- Alternative modes of transportation available
This traffic analysis evaluates total I-5 mainline volumes combined with express lanes volumes. The locations studied are Mercer St for northbound traffic and NE 45th St for southbound. The mainline has four lanes in each direction.
Dates without major sporting events were compared to dates with major sporting events to evaluate the traffic impacts of the events.
I-5 volumes on weekdays with a sporting event
Figure 2 below shows traffic volumes on weekdays with a major sporting event.
On I-5 northbound, volumes steadily increase during the morning commute and reach a capacity of 7,500 vehicles per hour (vph) by 7:30 a.m. At this time the roadway becomes congested and we see no further increases in throughput as shown by the horizontal trend between 7:30 a.m. and 11:15 a.m. This northbound congestion remains until 11:15 a.m. when the express lanes open to northbound drivers. During the afternoon commute, northbound traffic peaks at approximately 5:15 p.m. with 12,000 vph and begins to decline afterwards. The express lanes allow an additional 6,000 to 7,000 vph to travel northbound compared to southbound.
On I-5 southbound, traffic peaks in the morning commute at approximately 7:30 a.m. with 12,500 vph. Congestion increases on I-5 southbound at the Ship Canal Bridge when the express lanes service closes for the midday reversal at 11:00 a.m. As volumes increase in the afternoon, southbound capacity is exceeded causing heavy backups. This results in lower throughputs of vehicles, which is shown by the slightly sloped horizontal line between 11:15 a.m. and 3:10 p.m. in Figure 2. During the peak congestion period between 3:00 p.m. and 4:00 p.m., traffic breaks down even further resulting in a significant decrease in throughput. Southbound congestion lasts until 7:10p.m. when volumes start slowly declining.
I-5 volumes on weekdays without a major sporting event
Figure 3 below shows traffic volumes on weekdays without a major sporting event.
On I-5 northbound, traffic volumes peak at 5:10 p.m. with 11,700 vph.
On I-5 southbound, traffic volumes peak at approximately 7:10 a.m. with 12,200 vph. Southbound congestion lasts until 7:00 p.m. when volumes decrease at a faster rate.
The express lanes are closed for the midday reversal when southbound volumes have significantly decreased and when northbound volumes begin to increase.
Congestion builds on I-5 southbound on a typical weekday when the express lanes service closes at 11:00 a.m. for the midday reversal to northbound. This congestion builds and continues into the evening commute until 6:30 p.m.
The volume graphs show that weekdays with evening sporting events are similar to non-event weekdays.
Total directional volumes are shown in Figure 4 below. Northbound traffic volume exceeds that of southbound during the 11:15 a.m. to 7:30 p.m. commute.
Weekday sporting events occurring in the late afternoon and early evening hours around Seattle downtown area have similar traffic volumes as weekdays without sporting events for both northbound and southbound I-5, and do not warrant modifications to the express lanes service schedule.
Therefore, in order to equally redistribute and alleviate congestion around Seattle down town area, it is not recommended that the express lanes schedule be modified at this time.