This is a reasonable reply and I appreciate the very relevant data points.
I think you're correct that speed limits are designed knowing that most people will exceed them slightly. I'm curious if there are any studies that have addressed this? Eg. traffic surveys where they either raised or lowered a posted speed limit -- how much impact does that have on behavior?
Lowering speed limits by 5, 10, 15, or 20 mi/h (8, 16, 24, or 26 km/h) at the study sites had a minor effect on vehicle speeds. Posting lower speed limits does not decrease motorist's speeds.
Raising speed limits by 5, 10, or 15 mi/h (8, 16, or 25 km/h) at the rural and urban sites had a minor effect on vehicle speeds. In other words, an increase in the posted speed limit did not create a corresponding increase in vehicle speeds.
Or another way to state this is that people generally drive at a speed they think is reasonable given the road conditions and mostly ignore posted speed limits altogether.
No surprise to me, many states didn't have speed limits outside of town at all in the US until the federal government withheld highway funding until they posted speed limits. That's the reason the 55mph highway speed limit was originally established. It's also the same method by which we forced the drinking age from 18 to 21.
Given a population of people, some will be reasonable in their behavior, some will not be. Setting limits on that behavior is more of a signal (or suggestion) than anything that really changes how people behave in the large. To consider otherwise is to be naive of the reality.
Fun story, data shows that raising highway speed limits results in less accidents, but the accidents that do occur result in more fatalities.
I read their methodology[1] to determine when they measured the speeds, but it wasn't very clear when they did. However, I think a reasonable interpretation of their wording is that they measured just before and just after the change.
In my opinion there is a problem with this approach, in that many motorists will likely pay little attention to signs when going their usual routes, as they already know them all. If a speed limit is changed it may therefore take some time for people to notice that that has happened. To measure the full impact of the change, it seems the measurement would have to be taken a few months after the change.
There are some good studies that basically point to lane width , and road condition being determining factors on speed. E.g. narrower lanes, slower traffic.
Think of the speed limit as a fine tuning lever and the road itself as the control. If you slap a 30 mph limit on one of those 6-lane boulevards that you see a lot in Florida, there's little change. Likewise, the 70-75 limits on I95 in South Carolina don't really have a material impact on the traffic flow.
Road engineers can slow a road down with all sorts of techniques, and they absolutely do studies and instrument the road to tweak the design.
I think you're correct that speed limits are designed knowing that most people will exceed them slightly. I'm curious if there are any studies that have addressed this? Eg. traffic surveys where they either raised or lowered a posted speed limit -- how much impact does that have on behavior?