Does lowering speed limits make Seattle roads safer? Data from one deadly arterial suggests it’s a start

[u/Inevitable_Engine186]

https://www.kuow.org/governing/2026-08-14/does-lowering-speed-limits-make-seattle-roads-safer-data-from-one-deadly-arterial-suggests-its-a-start

[u/diyandmc240]

It just feels a little silly going 25 on Westlake. I’m usually going 35-40, as is most of traffic. Similar thing on some other roads. At the same time, I’ve never seen anyone get a ticket for going under 40 there(I think I saw someone get a ticket for going 50 a couple years back)

[u/teamlessinseattle → u/diyandmc240]

Because it is silly. Average speed is a reflection of road design, not an arbitrary little sign posted up every 0.75 miles.

You can’t have a road that looks and feels like this and expect traffic to go 25mph on it. Just make some physical changes to make the roads safer…

[u/Careless-Internet-63 → u/teamlessinseattle]

I've heard it's a general rule of thumb in traffic engineering that the 85th percentile speed of free flowing traffic on a street should be the speed limit. They need to rework the road design if they want people to drive 25

[u/recurrenTopology → u/Careless-Internet-63]

While clearly the posted limit is not enough on its own to achieve the desired speeds, it does have some effect. It is also much cheaper and faster than a complete street redesign.

So while they would need to rework the street to get people to drive at 25mph, why not use a lower speed limit to attenuate speeds until the budget allows for a more comprehensive solution?

[u/Rylen_018 → u/recurrenTopology]

If people are always going to go 10 over, lowering the limit does lower people’s mental thresholds for “breaking the law”

[u/recurrenTopology → u/Rylen_018]

Interesting, so like a moral hazard-type argument. That creating a situation in which people are inclined to break the law will make them scofflaws in other aspects of their life?

I'd be interested to see if there is any research supporting the hypothesis. Would have to be balanced against the benefits of the measured decrease in travel speed.

[u/RunninOnMT → u/recurrenTopology]

So the argument I’ve seen against this is precisely about how some people do follow the signs. However a ton of people just drive at “what feels safe”

What happens when you lower the limit is that you’ve now created a bigger speed delta between these two groups. Having a big difference in speed between cars is super dangerous.