[credit_guy]

They can't be just based on safety. If they were, then 5 mph is safer than 10 mph, and 2mph is safer than 5 mph.

Road speeds need to be a tradeoff between safety and convenience. Unfortunately, in our mind safety and convenience are not just quantitative things, they carry moral connotations. Safety is good, convenience is at best neutral.

Until people develop a good framework to express this tradeoff, speed limits will be more or less arbitrary.

[dazc]

There is a road on my route home that switches from 50mph to 30mph for no other apparent reason than it is slightly residential. Fair enough, but there are many similar examples where this isn't the case - the limit is uniform regardless.

What people miss about this is that there is also a slightly awkward bend at mid-way point and it isn't unusual to see a car lodged into the fence that runs alongside.

Sometimes the speed limit is based on statistical information that may not be obvious until one day you are the fool who thinks he knows better.

[dsfyu404ed]

Other than facilitating smug "we know better" commentary from the peanut gallery why not throw up chevrons?

It's long since been established that alerting drivers to specific hazards at or before those hazards is far more effective at reducing the hazard than dropping the speed limit or slapping up a "windy road" sign far in advance.

[dazc]

There is actually signage alerting people to this hazard, as is the norm, in addition to the lower limit.

Ironically, there is a low bridge a mile before this bit of road with huge signs alerting truck drivers that it is quite a bit 'lower' than they might be reckoning on.

Guess what, it still gets hit - so much so there is now flashing light signage in addition and, thus far, this has also proved ineffective.

I'll save the red traffic light signal example for later...

[tzs]

Sounds similar to the famous 8 ft 11 in bridge [1] (since raised to 12 ft 4 in, but still racking up victims). 177 crashes since 2008, most of them caught on camera at that site.

[1] http://11foot8.com/

[readthenotes1]

One of the things I think you are labeling under convenience is efficiency/productivity.

Until we can all work from home and use replicators to create our stuff, transportation will be a huge part of the production side of society. Speed limits affect that directly.

[mannykannot]

In the places where the speed of road transport has most influence on productivity, the speed limits are largely moot because of traffic congestion at the times when most commuting happens. In places and at times when congestion is not a problem, speed limits rarely have much effect on productivity.

[v01dlight]

I think the point about productivity is less about people's 'burbs to downtown commute and more about say, the trucking industry. Trucks drive at or near the speed limit for much of their long highway routes, and over those distances, a slight difference in average speed can mean a big difference in arrival time, and throughput of the whole system.