That’s the question one reddit user asked this week after noticing that traffic tends to increase through the work week but suddenly die on Friday.

Why tow trucks can’t get to a crash scene any faster

The person illustrates the “weird” phenomenon by ranking each day as such:

If I were to use numbers to illustrate, it’s something like 2, 3, 5, 8, 1. On Thursdays, traffic [is] much worse than normal and the buses and trains are … sardine cans. Why the huge differences in traffic? Why can’t you have normal, regular weekday traffic like other cities?

The answer, according to KIRO Radio’s Chris Sullivan, could come down to four-day work weeks. According to the Washington State Department of Transportation, Sullivan says, Friday can be particularly bad for drivers if there is a three-day weekend and people have Monday off — such as Labor Day. Most of the time, however, it’s predictably light.

Another reddit user’s theory backs that up, noting the large number of tech workers in Seattle.

West coast business culture in general combined with huge numbers of tech/knowledge industry workers with particularly flexible/variable schedules work from home, 10-6/11-7 types schedules, and so forth.

Plus:

There’s also a strong “reverse commute” (with respect to the usual American style) driven by people who live in the city going out into the suburbs for their day jobs. It’s important to note that because of Seattle’s geography, relatively small accidents can result in traffic grinding to a halt. A commute that typically seems “light” can quickly become a driver’s worst nightmare. Just take the crash on the 520 Bridge earlier this week as an example. One stalled bus slowed traffic for hours as drivers found alternative routes around or across Lake Washington. It’s just one example of what Sullivan calls the “ripple effect” that can occur on our roads.

Send Chris a traffic tip at @KIROTraffic on Twitter or email him.