I-5 express lane operation on weekdays

February 6, 2018

Express lanes concept

The Washington State Department of Transportation uses express lanes on I-5 to improve traffic flow during peak travel times. These lanes are opened in the direction with the highest demand in order to alleviate congestion as much as possible. The express lanes are operated on a weekday schedule and a weekend schedule. The schedule is sometimes changed to accommodate major events, construction work, or 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 downtown such as concerts, Mariners, Sounders FC, or Seahawks games.

Analysis data

Traffic data between June 2017 and November 2017 were used in this study. Days with and without major sporting events were included. The table below lists the dates used in the study. Days with sporting events were selected if they contained concerts, Mariners, Sounders FC, or Seahawks games that began between 12:00 p.m. and 7:30 p.m.

Dates without major events

  • June 5, 12, 14, 26, 29
  • July 10, 11, 12, 13, 14, 17, 18, 19, 27, 31
  • August 1, 2, 3, 17, 21, 25
  • September 7, 11, 12, 13, 14, 15, 26, 27, 28
  • October 2, 10, 18, 26
  • November 3, 6, 14

Dates with major events

  • June 6, 7, 8, 9, 16, 19, 20, 21, 22, 23
  • July 3, 5, 6, 7, 19, 20, 21, 24, 26
  • August 9, 10, 11, 14, 15, 16
  • September 1, 4, 5, 6, 19, 20, 21, 27
  • November 2, 20, 30

Although the report focuses on the overall results and the main factors that may affect operations, this express lanes study considered all of the following:

  • Northbound and southbound volumes at multiple locations
  • Northbound and southbound delay at multiple locations
  • Overall congestion
  • Access to and from the express lanes
  • Demand for destinations serviced by the express lanes, and
  • Alternative modes of transportation available

Traffic analysis

This traffic analysis evaluates total I-5 mainline volumes combined with express lanes volumes. The graphs shown use volumes just north of SR 520. At this location, 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 without a major sporting event

The graph below shows traffic volumes on weekdays without a major event.

On I-5 northbound, traffic volumes increase until 7:30 a.m. when the roadway reaches capacity at 5,400 vph. Northbound remains congested until the express lanes open in the northbound direction at 11:15 a.m. Volumes increase again until approximately 3:00 p.m. when volumes remain steady between 9,000 and 9,500 vph. The roadway remains congested until volumes decrease around 6:30 p.m.

On I-5 southbound, traffic volumes peak at approximately 7:30 a.m. with 12,000 vph. Southbound congestion decreases until the express lane reversal at 11:00 a.m. At this point, congestion builds again as volumes reach capacity and therefore overall throughput is decreased between 2:30 p.m. and 6:30 p.m. Speeds start increasing again at 6:30 p.m., which increases throughput.

I-5 volumes on weekdays without a major sporting event

 

I-5 volumes on weekdays with a major event

The graph below shows traffic volumes on weekdays with a major event.

On I-5 northbound, volumes steadily increase during the morning commute and reach a capacity of 5,500 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 reaches capacity at approximately 4:30 p.m. with 9,300 vph and remains congested until approximately 6:30 p.m. when volumes begin to decrease. The express lanes allow an additional 4,000 vph to travel northbound compared to southbound.

On I-5 southbound, traffic peaks in the morning commute at approximately 7:30 a.m. with 11,400 vph. Volumes quickly decrease as commuters arrive at work and therefore congestion is relieved. Congestion increases on I-5 southbound 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 downward slope between 12:00 p.m. and 3:00 p.m. in Figure 2. Southbound congestion lasts until approximately 6:30 p.m. when speeds start to increase which results in higher throughput, as shown by the upward slope between 6:30 p.m. and 8 p.m.

I-5 volumes on weekdays with a major sporting event

 

Conclusion

Currently, the express lanes are reversed as southbound volumes decrease and northbound volumes increase. Therefore, in order to equally distribute and alleviate congestion around the Seattle downtown area, it recommended that the express lanes schedule remains as-is.

Furthermore, 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.