April 1, 2008
On weekends, the I-5 express lanes run southbound from 7 a.m. to noon and northbound from 1 p.m. to 11 p.m. The purpose of this study is to see if extending southbound express lane service one hour, until 1 p.m., would benefit more drivers.
Traffic Study Summary
The study showed that during the worst congestion on a regular weekend southbound drivers were experiencing longer back-ups and more delays when compared to northbound congestion at its worst. We found that extending southbound express lane service until 1 p.m. more evenly distributes delays experienced by drivers traveling into Seattle on I-5 in both directions. While northbound drivers will experience a spike in congestion when the express lane reversal is delayed until 1 p.m. persistent southbound back-ups are reduced by one hour.
Study Data
In order to evaluate the affects on traffic of closing southbound I-5 express lane service at noon versus 1 p.m., traffic data from several weekends in 2007 was collected and analyzed. For the noon reversal scenario, we took data from days where there were no major events in the downtown area and we selected at least one date from every month of the year. For the 1 p.m. reversal scenario, we took data from days when the southbound I-5 express lane switch was delayed until 1 p.m. because of Mariner games at Safeco Field. These dates spanned from April to September.
Traffic Study
Below we compare the effect the noon reversal scenario and the 1 p.m. reversal scenario had on overall traffic back-ups and delays and on percentage of delay experienced by each direction of traffic.
Northbound Saturday traffic volumes
This graph shows northbound I-5 traffic volumes on the Ship Canal Bridge on Saturdays when the express lanes close southbound at noon and switch to northbound by 1 p.m. The mainline traffic volume peaks at noon reaching 7,000 vehicles per hour.
This graph shows northbound I-5 traffic volumes on the Ship Canal Bridge on Saturdays when the express lanes close southbound at 1 p.m. and switch to northbound by 2 p.m. The mainline traffic volumes peak at noon reaching 7,500 vehicles per hour.
In both scenarios congestion on northbound I-5 is at its worst just before the express lanes re-open northbound. However, in the 1 p.m. reversal scenario the mainline volumes remain high for about an hour longer than in the noon reversal scenario. Additionally, On days with a 1 p.m. reversal the total northbound volume is around 130,000 vehicles with about 109,000 vehicles traveling on the mainline. On days with a noon reversal, about 123,000 vehicles travel on northbound I-5 with 102,000 of those vehicles traveling on the mainline, this is roughly a 6% difference in total northbound traffic volumes.
Southbound Saturday traffic volumes
This graph shows southbound I-5 traffic volumes on the Ship Canal Bridge on Saturdays when the express lanes close southbound at noon and switch to northbound by 1 p.m.
This graph shows southbound I-5 traffic volumes on the Ship Canal Bridge on Saturdays when the express lanes close southbound at 1 p.m. and switch to northbound by 2 p.m.
The graphs show very similar traffic patterns between 7 a.m. and 10:30 a.m. However, after 10:30 a.m. in the 1 p.m. reversal scenario southbound traffic volumes begin to increase more quickly, reaching almost 9,000 vehicles per hour before the express lanes close. In the noon reversal scenario the total southbound traffic volume before the express lanes close is near only 8,000 vehicles per hour. On days where the southbound express lanes are closed at 1 p.m. the total southbound volume is around 117,000 vehicles with about 106,000 vehicles traveling on the mainline. On days where the southbound express lanes closed at noon traffic volumes reach only about 112,000 vehicles with 103,000 of those vehicles traveling on the mainline. This is about a 4% difference in the total southbound traffic volumes. The increased volume in both directions of I-5 during a 1 p.m. closure scenario is likely due to extra Mariner game traffic.
Northbound traffic delays versus southbound traffic delays in the noon reversal scenario
These graphs show how much worse traffic back-ups are for southbound drivers versus northbound drivers on days when the southbound express lanes close at noon.
When the southbound express lanes close at noon on a non-event Saturday, the congestion faced by northbound drivers is significantly less than the congestion for southbound drivers. Northbound traffic begins to get heavy around 10:30 a.m. and is immediately eliminated at around 12:45 p.m. when the express lanes typically reopen in the northbound direction. Heavy southbound back-ups start right when the express lanes close southbound at noon. This traffic gridlock extends from the Ship Canal Bridge to Northgate Way, approximately 3.5 miles, and lasts until around 7 p.m.
Northbound traffic delays versus southbound traffic delays in the 1 p.m. reversal scenario
These graphs show that traffic back-ups are slightly worse for both northbound and southbound traffic on days when the southbound express lanes close at 1 p.m.
On Saturdays when the southbound express lanes close at 1 p.m., both northbound and southbound congestion is slightly worse than when they close at noon. Some of this increased congestion is due to the increased traffic heading to downtown for Mariner games on these days. While the congestion for each direction is worse, we also see that the traffic congestion is more equally distributed between northbound and southbound traffic. Since the express lanes stay open one hour longer southbound in this scenario, congestion for southbound traffic does not start until one hour later but still ends at the same time, 7 p.m. In this scenario northbound traffic congestion significantly increases due to the later opening, however this period of congestion occurs over a shorter period of time than the southbound congestion.
Percentage of delay experienced by both directions of traffic in the noon reversal scenario
This graph shows the delays that occur on a typical Saturday when the express lanes close southbound at noon.
On Saturdays when the southbound express lanes close at noon, delays on northbound I-5 start to build around 10 a.m. while southbound traffic continues to be congestion free with the help of the express lanes. By noon, when the express lanes switch from southbound to northbound, northbound traffic congestion is at its worst with a maximum delay of 75 vehicle-hours (vehicle-hours is a measurement of the total amount of delay multiplied by the number of drivers experiencing that delay). When the express lanes re-open in the northbound direction around 12:45 p.m. northbound congestion quickly disappears and by 2 p.m. northbound traffic is experiencing almost no delay. The delay for southbound traffic begins right at noon when the express lanes close. The southbound delays rapidly increase, reaching a peak of roughly 92 vehicle-hours. When comparing the total vehicle-hours of delay southbound traffic is clearly experiencing more delays then northbound traffic. Percentage wise northbound traffic experiences only 25 percent of the total delay experienced by both directions and southbound traffic experiences 75 percent.
Percentage of delay experienced by both directions of traffic in the 1 p.m. reversal scenario
This graph shows the delays that occur when the express lanes close southbound at 1 p.m.
On Saturdays when the southbound express lanes close at 1 p.m. delays on northbound I-5 start to build around 10 a.m., while southbound congestion frees with the help of the express lanes. The northbound delay continues to increase until the express lanes open northbound around 1:45 p.m. Because delays for northbound traffic are longer in this scenario it takes northbound traffic longer to recover once the express lanes open northbound. Southbound delays begin to increase when the express lanes close southbound and delays continue to remain high until 7 p.m. Due to a higher total daily volume from traffic headed downtown for events on these days, there are more delays experienced in both directions. Comparing the delays experienced by both directions, percentage wise northbound traffic now experiences 45 percent of the total delay experienced by both directions and southbound traffic experiences 55 percent.
Sunday traffic delays in the noon reversal scenario
Sunday traffic delays in the noon reversal scenario
These graphs show how sensitive Sunday traffic is to switching the timing of the express lanes.
Sunday traffic is on average much lighter than Saturday traffic. In the noon reversal scenario, northbound traffic never gets a chance to significantly backup; while southbound traffic experiences considerable backups when the express lanes close at noon. In the 1 p.m. reversal scenario the opposite is true. Northbound traffic experiences significant delays prior to the reversal of the lanes at 1 p.m. while southbound traffic delays are much smaller after the reversal. On noon reversal days there was a total of 3,200 vehicle-hours of delay with 84 percent of the delay experienced by southbound drivers versus 16 percent of the delay experienced by northbound drivers. On 1 p.m. reversal days there was a total of 3,000 vehicle-hours of delay with 41 percent of the delay experienced by southbound drivers versus 59 percent experienced by northbound drivers.
Conclusion
In the noon reversal scenario, the northbound delay on Saturdays is fairly moderate when compared to the southbound delay which begins at noon and persists throughout the day. In the 1 p.m. reversal scenario the northbound delay grows significantly worse as it spikes just before the reversal while the southbound delay is alleviated by the extra hour of southbound express lane service. This spike experienced by northbound drivers is undesirable, but it helps equalize the total delay experienced by northbound and southbound traffic. Comparing the two scenarios, the persistent congestion experienced by southbound traffic in the afternoon in the noon scenario is more difficult for drivers to avoid than the short spike in delays experienced by northbound traffic in the 1 p.m. scenario. Therefore, this report finds that the 1 p.m. reversal on Saturdays equalizes the total delay experienced by either direction of traffic on I-5 through Seattle.
Sunday congestion is much more sensitive to timing of the express lane reversal than Saturdays. When the express lanes change at noon, the southbound delay is significantly worse than northbound delay and the exact opposite occurs when the lanes change at 1 p.m. Because both scenarios create a similar outcome but favor opposite directions the Sunday schedule should match the Saturday schedule.