[revelation]

Increased speed saves time only linearly but requires a with the square increase in power to overcome wind resistance, the major factor. With the power increase comes a similarly large increase in crash damage and mortality particularly for pedestrians. This is in addition to the increased crash risk simply due to the increase in speed while mental capabilities of drivers remain constant(ly poor).

See the data here:

http://humantransport.org/sidewalks/SpeedKills.htm

As this makes clear, even 30mph is too fast for a residential area in the sense that the tradeoff makes no sense; 20mph brings a massive reduction in deaths but only a comparatively much smaller increase in travel time (in a grid city with constant lights anyway).

[vcarl]

And having speed limits set too low for the road leads to a mismatch between posted risk (the speed limit) and actual risk (the actual speed of cars) to pedestrians. There's evidence that speed limits have no impact on how fast people actually drive[0], so posting a lower speed limit is less likely to have an impact than designing roadways for a certain speed in the first place.

[0]: https://www.ibiblio.org/rdu/sl-irrel.html

[revelation]

Hence the need for speed enforcement. We've come full circle.

[vcarl]

Designing roads properly for the desired speed removes the need for enforcement. And enforcement doesn't mean that people go the posted limit, it just means more tickets.

[sandworm101]

Nope. All sorts of vehicles drive on roads. A motorcycle will always be able to navigate a road far faster than anyone intended. Roads need some sort of speed limit to specify unseen dangers, to communicate to the motorcycle that there is a reason the designer doesn't want him moving at the speed of which he is physically capable.

[dalke]

Yes, there will still need to be speed limits. But if, say, the roads are designed with a 12' lane, the people will tend to drive faster than on a 10' lane. (See http://www.citylab.com/design/2014/10/why-12-foot-traffic-la... .)

I believe vcarl's point is to design the roads for the speed limit, so the speed limit matches what people will perceive to be the right speed for the road. Don't have speed limits which are lower than the perceived limit.

The clumsy way to do this is to put in traffic calming devices. Narrowing the streets is less in-your-face, and comes with other advantages, like cheaper road maintenance. Both are also ways to communicate the appropriate speed limit, in addition to a sign which says the same thing.

[sandworm101]

But setting a limit based on roads doesn't work once we realize that every road is different. We'd need signs everywhere. So we set standard measurements (50kph cities/ 80-highways, special signing for anything else) and build to those standards where practical. So a few roads get slower speeds than perhaps they should, but the standardization brings advantages. Imho good cops should then take it upon themselves to enforce those standards where most appropriate.

Traffic calming isn't about slowing people down. It's about making driving on street-A so painful that people go to street-B. I really like those little roundabouts they use instead of 4-way stops. A motorcycle can basically take a strait line from one side to the other without slowing down. Far easier than a stop sign.