Why is the speed limit where it is?
Wouldn’t it be safer were it slower?
Sure but nobody likes driving 10 mph (16km/h for you metric types ;)) on the highway.
I’d venture to say that most accidents are the result of people exceeding the posted limit and being reckless (e.g., driving and talking on the phone but that discussion is probably best left for a different thread).
Yes, it would be safer if everybody drove slower, but setting the speed limit slower would mean that some drivers would drive at that limit, and other drivers would still speed. Those that were used to the old limit and disagreed with the new one would be frustrated and angry at the people driving at the new speed, and that would cause accidents.
The current limits are a balance between the need for safety and the need (private, public and commercial) to get somewhere within a satisfactory time.
There is an argument for lowering and strictly enforcing speed limits in particular areas (near schools etc.), but generally speed limits are a balance of safety and convenience. The inevitable comparison that someone will now make (so I’ll make it first) is (was) the convenience, and therefore relative low security, of internal US flights.
It probably would be safer if people drove slower; it would be safer still if people didn’t travel at all, but compromises have to be struck.
I believe that the speed limit is geared more towards fuel economy and air quality than safety. At too low a rate of speed, your engine would run uneconomically because your engine is running longer and taking you a shorter distance. At too high a rate of speed, your engine would burn too much fuel, wear parts, wear roads, etc. You also have to remember that cars today are designed to be economical at the current speed limit. The gear ratios are set so that the engine runs at it’s most efficient RPM at the common national speed limit. This design practice is one thing that has helped to keep speed limits around the country close to what they were 50 years ago. Hope that helps.
What is important is not speed as such, but speed that is poorly adapted to external conditions. In Germany there is no speed limit on (most of) the ‘Autobahn’, and the average speed, as well as traffic density is high. Yet they only have an accident rate of 7 accidents per billion vehicles and km. The corresponding figure for rural roads is 24.
Another interesting fact is that when the speed limits on Swedish motorways were decreased by 10 and 20km/h in the 1970s, the mean speed of traffic decreased only by 3 to 4 and 6 to 8 km/h, respectively.
These and other facts about the effect of speed on traffic can be found in FHWA Study Tour for Speed Management and Enforcement Technology. Well worth a read! (as is this thread)
Also, keep in mind that not everywhere has the same speed limits. In Montana, the slowest highway speed limit is 70 mph, and that is on two lane highways. On the interstate, it’s 75 mph. Studies have shown that people in Montana tend to drive 3-5 mph over the speed limit on average.
We had a lower limit back when the national limit was 55 mph. Everyone still drove 75-80 mph (the fine was $5 and no points off your drivers license). There were more accidents at that time (pre-airbag and mandatory seat belt laws). No one wanted the lower speed, because of the distances we often travel in this state.
This week I have a 850 mile round-trip drive for one 6 hour meeting. At 55, that’s 15.5 hours of driving. At 75, it’s 11 hours. That extra 2 hours of driving on each leg seems more dangerous to me that driving faster.
BTW - In 25 years of driving, the scariest time was when Montana most recently had no real speed limit. There was no MPH limit, just “reasonable and prudent”, as determined by the highway patrol. Gramma’s doing 55 in the right lane, I’m doing 75 and pull out to pass and some jerk comes flying up behind me at 100 mph. It was pure chaos.
In abstract, yes. We would all be much safer if we travelled a lot slower.
However, I travel about 2 hours on a freeway each day, getting to and from work. The posted speed is 110 km/hr. (I don’t do conversions, but it’s probably around 70 miles/hr)
There are quite horrific accidents on this stretch of road, but they mostly occur during holiday times, when they are overloaded and driven on by people inexperienced at driving at these speeds. An accident during commuter hours is rare. One particular blackspot (high accident area) has now had the speed reduced to 90 km/hr (about 55 miles/hr) and causes a lot of problems, due to the fact that people have to adjust their speed, and different people adjust it to different speeds. I for one, continue travelling at at least 100 km/hr through this section, as do the vast majority of commuters. On the weekends however, I slow to 90 km/hr since almost everyone else does, and were I to insist on travelling at 100 km/hr, I feel I would be more likely to contribute to accidents happening. The accident rate for that stretch of road, by the way, has increased by 14 %. Therefore, sometimes, travelling slower can be more dangerous than leaving well enough alone.
But, yes, in a lot of cases, travelling slower is safer. Just don’t increase my commuting time any further by making me slow down !!! 
one thing not posted yet, in america the freeways are engineered to be driven at 70, people are comfortable driving them at 70, and well that about says it. people will tend to drive at comfortable speeds regardless of the speed limit.
in UK it is 70mph. in Germany on Autobahns there is no speed limit. you often get Porsches flying past you at 160mph.
I wouldn’t say that 160mph was often. But it’s not unusual to check your mirror and suddenly find a large black car screeching up behind you that was simply nowhere seconds before. One thing they do have to say for Autobahns; they are excellent roads that put most in the UK to shame. I can’t imagine many circumstances you’d be able to do 160mph on a motorway.
Although there is no speed limit you can still get done for dangerous driving if you were driving at excessive speed for the conditions. So it’s more of a kind of flexible speed limit. You could, theoretically, get charged for driving at 50mph in rain, or 40mph in heavy traffic. German fines and penalties for traffic offences are heavy. It’s also common for your insurance to be voided if you crash at high speeds, regardless if you were at fault or not.
So autobahns are not quite the free-for-alls they are often portrayed as. Plenty would argue that this model for speed limits is safer than fixed limits.
Yes. Read the link I provided above. Quoting:
So, while the high speed is (or can be) safe, unadapted speed is dangerous.
The most common UK urban speed limit is 30mph. Several years ago (sorry, no cite - too long ago) I read that 19 out of 20 people hit by a car at that speed survive, whereas 19 out of 20 people hit by a car at 50mph die, and that that is the reason for the 30mph limit.