[zanny]

I live in a town here the main street has a posted speed limit of 25 that absolutely nobody obeys and the average speed of traffic is always around 40.

If you take a road with a "natural" speed and try to artificially restrict with a speed limit, unless you are prepared to have officers posted on that road at all times ticketing people will ignore the limit entirely and go the "natural" speed of the road.

I see it all the time, all over, especially since I live in PA where the state thinks its a really sensible idea to keep almost every highway at 55 mph no matter what. When you try to heavily constrain car speed well below the natural speed of the road people simply stop trying to obey the limit at all and go whatever speed they want.

[clairity]

i wish this were more common knowledge. narrowing lanes is among the best ways to slow cars down, if that's the goal.

but in many cases, that goal is missapplied to reduce accidents. accidents are typically not caused by speed, but rather distraction or anger. it hard to enforce attention and mindfulness, so we regulate speed as a (poor) proxy (partially for harm reduction, as speed increases severity of accidents), which directly leads people to wrongly associate speed as the cause of accidents.

it makes sense, for example, to reduce vehicle speeds around schools to reduce harm in case of accidents with small people. but rather than an artificial speed limit that depends on police enforcement, narrow the lanes to 8 feet and people will naturally drive 15-20 mph in those school zones without the added enforcement burden (and use the remaining road space for bike lanes).

[emj]

Lower speed leads to fewer deaths; at ~18 mph almost no one dies, compared to 80% of pedestrians who are hit at 30 mph die. Your reasoning comes from another angle it might be correct but you can never assume people are attentive in traffic, neither pedestrians nor drivers, that's why you need rules and infrastructure that makes it possible to share the roads.

[ryantgtg]

In California (and I believe many other states), what you describe would be considered a speed trap, and it isn't allowed. Speed limits are supposed to be set by measuring how fast people drive on the roads. If 85% of drivers go 40, then that's the speed limit. If a city/county hasn't surveyed a street recently, then an officer cannot park their vehicle and hand out tickets for speeding (though they can give a ticket if they are driving their vehicle and see someone speeding) - because that could be a speed trap.

In 2016, the speed limit was enforceable on only 19% of streets in Los Angeles due to the speed trap law. The city has made great efforts recently to update their speed surveys, resulting, in most cases, in an increase in speed limits.

[amanaplanacanal]

The roads are engineered wrong. If you design a road for 40, then set the speed limit at 25, it's not going to work. You have to re-design the road for the 25 MPH speed limit. There are several ways to do this, and they are well known.

Of course cities are chronically underfunded for infrastructure, so this is really hard to do in practice.