During last month’s weeklong Interstate 5 Bridge trunnion replacement project, it quickly became clear that Clark County commuters were doing a pretty good job of staying off the freeway. Even though both traffic directions had to share just three lanes on the southbound span of the bridge, it appeared that congestion was moderate at worst.
There wasn’t any data to pair with those observations until this week, when the Oregon Department of Transportation released its official count of the daily vehicle crossings in September. The data bears out what many drivers probably observed — traffic was less than two-thirds of normal levels during the closure week.
The process of closing the northbound span and shifting northbound traffic to the southbound span began late in the evening on Friday, Sept. 18, and was completed on the morning of Sept. 19. Crews began to reopen the northbound span at about 11 p.m. Friday, Sept. 25.
In the weeks leading up to the closure, total daily crossings ranged from about 103,000 to 139,500 on weekdays and from about 74,500 to 121,000 on weekends, with some of the peak traffic around Labor Day weekend.
Northbound and southbound span traffic each tend to comprise about half of the total number for a given day, although it’s not a perfectly even split. Daily southbound span crossings averaged 58,573 for the first 18 days of September, while daily northbound span crossings averaged 57,132.
During the closure, southbound daily crossings dropped to an average of 33,279 — about 57 percent of the average number during the first 18 days of the month.
Given that the average crossing counts for each direction tend to be relatively close, it’s likely that northbound traffic saw a roughly similar decline — but ODOT traffic data analyst Simon Peralta said there’s no official count for the northbound traffic that shared the southbound span’s lanes.
At least some of those missing drivers must have taken the week off, ridden transit or worked from home, because there’s only one other place where they could have crossed the river. The Interstate 205 Bridge saw a marked increase in traffic during the trunnion project week, but not enough to fully account for the decrease in vehicles on I-5.
Total daily crossings on I-205 averaged 136,159 during the first 18 days of the month. The average jumped to 164,903 during the trunnion week, then fell to 149,731 in the final five days of September.