Are road speed limits based on safety? | Hacker News Reader

Are road speed limits based on safety?

cricetuscricetus.co.uk

19 pointstransportguy23 comments

The implicit point here is that speed limits should be based upon safety, or they should be abolished. This article fails to mention that there are important reasons for speed limits other than safety. For example, noise, pollution and climate. Mobility and freedom of those without a car, especially the young and very old. Speed limits are a tool that can be used to help boost the health, fitness and quality of life for people who live and work near roads (that's pretty much all of us).

The point of all of these arguments is always that speed limits should be increased when all evidence suggests they should be lowered.

Evidence, in this case, being number of deaths.

Did you read the article? The first evidence it examines is roadway deaths vs speed limit, and the author concludes "there is little correlation". I'm not saying I agree with the article's conclusions, but it's easier to trust someone who shows some evidence than someone who just suggests that evidence which supports their conclusions exists.

I just take 'little correlation of deaths v speed limit' as an argument against speed limits in general. Otherwise, what is the point of raising the issue?

Also, he says: '10 times as many people die on UK country lanes than on motorways. This furthers our evidence that motorway limits have less effect on deaths than other roads..'

Comparing country roads to motorways? A country road could be one car width or two car widths at best, with vehicles driven, often, erratically in both directions with little warning of obstructions or oncoming traffic.

A motorway is 3 or 4 lanes of vehicles travelling in the same direction with virtually zero chance of anything coming the other way, usually clear visibility and advance warning of obstruction, etc

You don't see a problem here?

My memory of years ago is that comparing the survival rates of someone hit at 30mph vs someone hit at 50mph was a part of every driving test. And unless my memory is very wrong, there is an extreme correlation there.

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.

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.

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.

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.

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.

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.

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...

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/

My dad is a highway engineer in the US, has been for 40 years. He has always told me that their plans' speed limit recommendations are based on 5 mph less than the max safe driving conditions for the particular road. There are a lot of factors that go into max safe driving conditions (in ideal weather).

Curvature is certainly one, remember that different cars handle turns differently and the bank of the road can allow for faster turns than a non-banked road. Highways have a significant bank, but that depends on region because freezing conditions limit the ability to bank a highway (imagine stopped traffic on an icy road, you can't have them sliding toward the median!).

Another important factor is distance-to-stop. In some cases there is a median blocking the view around a corner. All else being equal the turn should be fine at 65mph but because of the lack of sight distance they have to lower it to 55mph.

Now the actual speed posted on the road doesn't always exactly match the engineering designs. For example, the PA turnpike recently increased the speed limit for most of the highway from 65mph to 75mph. Over the last 50 years they've done a lot of incremental enhancements on flattening turns, adding a 3rd lane, etc. but my dad has told me that the increase to 75 was dubious due to turning radius still being unsafe at 80mph over certain sections.

Another guess I should mention for the vast straight highways of the midwest is that drag increases as a square of the speed. Normal vehicles are more efficient at 60mph than 80mph so this _might_ be a factor in areas where a max safe driving speed approaches infinity (Kansas).

> In the UK the 60mph country roads in particular shouldn't be trusted to provide a safe speed at all times.

Especially on country roads, you shouldn't view the 60 mph speed limit as a speed target - drive at the speed appropriate to the road and conditions.

There's a big cultural difference between the US and UK driving signage. In the UK there are fewer signs and it's generally upheld that the speed limit is a limit, and that what constitutes lawful driving may require driving much slower. You're doing to have a hard time suing anyone if you lose control of a car on a tight and narrow bend on a country road but were doing 59mph (i.e. within the limit).

Contrast the US where little yellow signs on every bend suggest a speed at which it's safe to go around, usually a conservative one. I think something missing from US road signage is the idea that more is worse: drivers are trained to ignore signs and speed limits because there are so many verbose and pointless signs around. It's much harder to trust the installers of the signs when they're obviously silly. In the UK it's more as if signs likely indicate some legitimate hazard that a driver will find out about very soon.

This article bakes in some false assumptions about compliance.

You can't just set a 40mph speedway on a limited access highway because "tHaT's WhAt'S sAfE gUyZ". Drivers will ignore it. Except that one guy with a dead hooker or brick of cocaine in the trunk or some other specific reason to be very cautious about complying with the law. He'll do 40. And safety will have been reduced because you have increased the variance in speed of traffic on the road.

There's a reason that current civil engineering doctrine is to set speed limits based on traffic speed and not the other way around.

People who obey speed restrictions have a dead hooker in the trunk, who knew?

No, people with a dead hooker in the trunk obey the speed limit.

Completely different thing and an important distinction.

Roads do a surprisingly good job of making clear how fast is safe to drive on them. I'm sure there must be places where people think, "Oh wow, going with the flow of the rest of these drivers feels really unsafe," but much more common is the experience where people are comfortably driving 10-20mph over the speed limit and there is no higher incidence of accidents there than anywhere else.

I think in general setting a speed limit to try to regulate speed for the sake of safety is one of the least effective ways to bring about the desired change.

This logic can only make sense based on the axiom that almost nobody can be coerced into compliance. Sure, if fixed cameras are somehow impossible and the best hope for enforcement is "no more than 10 over no matter how bad the conditions" this may be the case. But in some countries other than the US these are solved problems.

I would say no they are not and that's because after driving for about five years a driver acquires an expert sense of the safe speed for the stretch of road they happen to be on. In fact sometimes the speed limits are set at close to the 85 percentile speed of the of the traffic, so in effect the drivers set the speed limit.

Snow on the road? No one is driving close to the limit. In my area there are even curves that are purposefully slanted away from the curve of the road so that drivers will slow down on that curve. For a lot of drivers their slowing is subconscious but it still works.

A wide, straight road beside an elementary school and traffic will fly by unless the serious steps are taken to alert the subconscious brains of the drivers to slow down. Narrow the road, put a huge red square across the road with a huge "20 mph" written in it and some drivers will slow down. More will slow down if there are kids around. Almost all drivers speed is set by their subconscious.

There is one particular situation where the driver's subconscious expertise doesn't work very well and that's on highways with intersections. It's raining, the light turns red, you're driving at a speed that's safe for the highway but not safe for stopping at the light and you rear end the car in front. On such highway near me they put in a long strip of high traction pavement in front of these intersections so when the light turns red drivers can stop safely and quickly.

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