The I-5 express lanes typically run southbound from 7:00 a.m. until noon and northbound from 1:00 pm to 11:00 p.m. on weekends. Currently, for games with a 12:30 kickoff, I-5 Southbound Express Lane service is extended until 12:30 p.m. We receive questions from the public asking if this schedule is appropriate for Saturdays with afternoon Husky Football games. We recently conducted a traffic study to answer this question.
Traffic Study Conclusion
After an inspection of the available data, we determined that there was no significant change in the traffic patterns on I-5 due to Husky games. The existing schedule of delaying the I-5 express lanes traffic switch by 30 minutes is adequate for the present conditions.
Study Data
There were four games from the 2006 season and one game from the 2007 season that we chose to analyze because they began at around 12:30 pm. In addition to the five Husky game dates, five Saturdays during the same time of year without Husky games were also included in the study to determine whether traffic patterns on I-5 were significantly changed during these football games.
The dates selected were:
| Games |
No Games |
| Sept. 2, 2006 |
Sept. 9, 2006 |
| Sept. 16, 2006 |
Sept. 20, 2006 |
| Sept. 23, 2006 |
Oct. 21, 2006 |
| Nov. 11, 2006 |
Nov. 4, 2006 |
| Sept. 8, 2007 |
Sept. 22, 2007 |
Traffic Study Summary
To determine if the current 12:30 p.m. to 1 p.m. reversal from the southbound to northbound direction on game days is appropriate, we retrieved and examined traffic volumes and occupancies from data sensors in the roadway. Occupancy is the amount of time a vehicle spends on one spot in the roadway. Additionally, we observed the traffic patterns that occurred on I-5 during the express lanes reversal to determine how the north and southbound backups responded to the switch.
Figure 1 (below) represents the traffic breakdown of the typical I-5 mainline in both directions at the Ship Canal Bridge on non-game days. This volume versus occupancy, or volume versus the amount of time a vehicle lingers on one spot of the freeway, relationship is fairly linear when the mainline free flow conditions exist. This linear relationship shows that breakdown occurs right around 6000 vehicles per hour. At that point, the occupancy begins to increase at a higher rate than the volume, which results in congestion on the roadway. From this graph it is clear that the amount of vehicles per hour the roadway can handle is approximately 6000 vph.
Figure 1: Volume vs Occupancy Relationship for Typical Saturday on I-5
As seen in Figures 2 and 3 (below), the traffic volumes on both the northbound mainline and the southbound mainline of I-5 hover near 6,000 vehicles per hour on Saturday afternoons. The volumes on the southbound mainline steadily increase throughout the morning while the express lanes run in that direction. At around noon, the southbound express lanes are closed in preparation for the switch to northbound. On both game days and non-game days this closure causes the southbound mainline to surpass the 6,000 vph volume and traffic begins to backup near the Ship Canal Bridge.
Northbound mainline volumes also increase steadily throughout the morning. The northbound volumes surpass 6,000 vph between 9 a.m. and 10 a.m. on both game days and non-game days and causes traffic to slow near the Ship Canal Bridge. These northbound volumes continue to build until around 1 p.m. when the express lanes open northbound. At that time the traffic volumes on the northbound mainline begin to decline and continue to decrease the rest of the day. Looking at the mainline alone, Figures 2 and 3 indicate that there is no significant difference in traffic patterns on I-5 during Husky Football Saturdays.
Figure 2: Volume vs Time on I-5 near Ship Canal Bridge on Non-Game Saturday
Figure 3: Volume vs Time on I-5 near Ship Canal Bridge on Husky Game Days
While the mainline volumes may indicate that there is no significant change in traffic on game days, there is still a possibility that the express lanes may be carrying additional traffic. Figures 4 and 5 (below) compare traffic on the express lanes and mainline I-5 on game days and non-game days using traffic volumes taken from I-5 northbound at the Ship Canal Bridge. On both game days and non-game days, the express lanes allow northbound volumes to increase by over 2,000 vph. Additionally, Figure 4 indicates that traffic volumes in the express lanes were actually higher for a longer period of time during non-game days.
Figure 4: I-5 Northbound Mainline and Express Lanes on Non-game Days
Figure 5: I-5 Northbound Mainline and Express Lanes on Husky Game Days
Figures 6 and 7 (below) also look at express lane volumes during Husky Saturdays and non-Husky Saturdays. Looking at southbound express lane volumes in the morning, it can be seen that volumes increase earlier on game days. Additionally, the southbound express lanes also appear to maintain higher volumes for a longer period of time on game days. However, as crews are about to switch the express lanes volumes begin to decline. These graphs indicate that the current closure times are appropriate because the southbound express lanes service the extra volume brought by the game and they close as this volume begins to decline.
Figure 6: I-5 Southbound Mainline and Express lanes on Non-game Days
Figure 7: I-5 Southbound Mainline and Express Lanes on Husky Game Days.
While the traffic volumes seem to indicate that traffic on I-5 is not significantly affected by Husky games, there is a possibility that traffic backups are occurring in areas on I-5 that were not considered in previous sections. To examine this possibility, traffic congestion map archives were retrieved for all of the days included in the study. Three flow maps from each day were studied to evaluate traffic on I-5 just after the southbound express lanes were closed, just before the northbound express lanes were opened, and 30 minutes after the northbound lanes were opened. Figures 8, 9 and 10 include the maps from September 22, 2007 with no Husky game and from November 11, 2006 which had a 12:30 pm Husky game.
Figure 8 (below) illustrates that right after the southbound express lanes closed, there was no traffic congestion on the southbound mainline approaching the Ship Canal Bridge. At the same time, the traffic on I-5 northbound has backed up about 7 miles from SR-520 to Corson Ave S. While the northbound traffic congestion is fairly bad, it should be noted that the game day and non-game day maps show extremely similar traffic patterns.
Figure 8: Traffic on I-5 Just After the Southbound Express Lanes Close
Figure 9 exhibits the worst of the northbound traffic backups because it was taken just before the express lanes open northbound. For the non-game day map, this backup extends 8 miles from NE 45th St to Corson Ave S. The game day northbound traffic backup has not extended beyond what it was in Figure 8, but it has become slightly more congested. Additionally, Figure 9 shows the beginning of the southbound backup on the mainline that begins to occur when the express lanes close.
Figure 9: Traffic on I-5 Just Before the Express Lanes Re-open Northbound
As indicated in the earlier volume analysis, Figure 10 (below) verifies that the traffic backups on I-5 northbound dissipate quickly once the express lanes open in that direction. On the other hand, without the added capacity of the southbound express lanes, the southbound mainline has backed up about 3 miles from the Ship Canal Bridge to Northgate. While this traffic is fairly bad and has backed up quickly, it should be noted that this 3 mile backup is less severe than the 7 and 8 mile backups in the northbound direction prior to the switch. It is important to again note the similarities between these two maps, one of which is from a game day and the other is not.
Figure 10: Traffic on I-5 30 Minutes After the Express Lanes Open Northbound
Both the traffic volume analysis and the traffic pattern observations indicated that Husky football games have only a minor effect on the I-5 mainline. The traffic patterns from the archive maps were nearly identical for game days and non-game days and the volume analysis only indicated a slight increase in volume on the I-5 southbound express lanes in the morning when fans commute to the game. Additionally, this slight increase in the traffic traveling in the southbound express lanes dissipated just before the switch to northbound began. At this time it seems that the existing schedule of operating the southbound express lanes until 12:30 p.m. and then switching them northbound is appropriate for Husky football Saturdays.