Some highways post different speed limits for trucks.Does anyone know why?
I don’t have a definitive answer, but I can hazard a couple of WAGs based on the assumption that trucks are heavier than other motor vehicles.
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On curves in the road, friction between the tires and the road helps keeps the truck in the turn. However, frictional force, while proportional to the weight of the truck, is only a percentage of the weight, so heavier vehicles might need to travel more slowly on curves to prevent them from sliding out of the turn and slamming into the guard rails.
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Perhaps trucks are less maneuverable than cars so forcing the trucks to drive more slowly gives truck drivers the needed time to maneuverve out of the way of dangerous situations.
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A truck crashing into something at 70 MPH does a lot more damage than a truck crashing into something at 40 MPH. Granted, cars crashing at 70 MPH do a lot more damage than cars crashing at 40 MPH as well, but trucks are generally more massive than your average four-door family vehicle.
I always thought it was because of their size. I assumed that the states considered those trucks unsafe at certain speeds. It takes longer to stop, and they are also affected more by wind the faster they go. If you ever rent a big U-Haul, you’ll find that you can’t drive as fast as you usually do, because it’s too hard to control. Just a WAG.
Woops. What Caldazar said. I need to work on typing faster.
I suppose the rationale is along the lines of what others have said above. But I have never been convinced that having lower limits for trucks was actually safer overall. It means that there is going to be a lot more passing (by cars of trucks) than would be the case if all vehicles had the same speed limit. Passing on the intersate highway is not very dangerous compared to passing on a two-lane road, but it probably quite a lot more more dangerous than driving the interstate without having to pass. I wonder if there are highway accident statistics that can clear up whether having lower speeds for trucks is safer overall.
I have cousins who have CDL’s and drive semi’s–I seem to remember one of the them saying if you drove the speed limit in them you’d get run off the road. In practical experience, it seems a good number of truckers do exceed those separate limits to keep up with the flow of traffic, and probably for good reason, as I can see how it could be extremely difficult driving in 80+ mph traffic doing 55 with a large vehicle. I have to think those laws probably aren’t enforced too heavily, except maybe in Ohio…
“one of them saying” that is… I don’t usually stutter IRL…
Trucks need a longer distance to brake than cars - due of course to their weight, but also to the fact that air brakes take up to 0,5 - 1 second from the time you floor the pedal to the brakes are effective - unlike a car’s hydraulic brakes that works almost instantly. The difference is quite noticeable (yes, I have a truck license).
Having vehicles with wildly differing braking distances driving close to each other has unpleasant consequences in case of accidents. Reducing the top speed of trucks is probably an attempt to even this out somewhat.
The logic might be flawed, as bibliophage points out, but how to prove it ?
BTW, are busses excempt from truck speed limits in the US ?
S. Norman
I heard from an Allstate insu. guy that it was needed to hold back the truck speed limit to 55mph to pass the 65mph law for cars. also I think that the state keeps the truck speed limit lower, so when they do nail them for speeding they make more $.
i personally think that the diffrence in speed is more a cause of accidents then the actual speed.
The last time I rode Greyhound, the driver stayed well ahead of the semi’s. Of course, that doesn’t necessarily make it legal.
Until someone has some solid info, it might help to consider whether they used to have truck speeds back before they dropped the highway speed to 55 in the 80’s. I can’t remember that far back (or at least not highway signs) and I don’t seem to recall seeing a seperate speed for trucks until they raised the speed limit again.
In Ohio and Kentucky at least, buses are exempt from the 55MPH speed limit – the signs for the 55 limit always read ‘Trucks over such-and-such weight and non-comm buses.’ So I guess commercial buses can go as fast as cars.
Caldazar was close in his response – the biggest reason for making trucks travel more slowly is turning capacity. The limiting factor in how steep a turn a vehicle can make isn’t sliding, but rolling over – and the high center-of-mass of trucks makes them more prone to tip over than shorter, wider vehicles (same reason SUVs roll so often in accidents). The curves on highways are banked to counteract this effect, but to keep trucks to the same margin of safety as cars, they must travel more slowly.