Reimpose the 55mph speed limit!?

Ugh. Wouldn’t get to enjoy my toys much then.

The US interstate system is designed for 80 MPH (though that doesn’t work too well when folks are on the phone, eating, doing their hair, etc).

At least there are decent mountain driving roads nearby in my case (north Georgia, southern Tennessee and North Carolina). Had lots of fun taking the new car on Tail of the Dragon last month. The guys & gals driving WRX STIs were probably having the most fun that day since some of the roads were under construction & covered in gravel. I was tip-toeing along trying not to kick up too much road junk while they were sliding around the corners rally-style…


Huh? Do you think the engineers carefully calculated the speed needed to keep cars on the road for a given embankment?

“Johnson… do you realize that if cars only go 65 mph on this curve, they’ll slip into the canyon below???”

“It’s OK, sir. This road has been designed for 80 mph. Lawmakers would be fools to tinker with our master plan.”

Little did Johnson & his boss realize the fools that would occupy the legislation.

That loop-de-loop north of Sacramento can only be taken at 65 mph or faster.

No, wait, that was Hard Drivin’.

Actually, I think they do.

I was under the impression that it was more about wind resistance.

That was my thought too, resistance is squared to it starts increasing really fast.

It’s funny, when I was making my move to Ohio, my grandfather suggested that I drive 55 the whole way. This would be “safer and give you better gas mileage”. Just for fun, I calculated that the trip would take almost 3 hours longer than if I drove it at 75. Now, I’m no expert, but I’m guessing 3 extra hours of driving more than cancels out the improved gas mileage.

Eastbound and down, loaded up and truckin’,
We’re gonna do what they say can’t be done.

You don’t know what mileage is. If you get better mileage, then a trip over a certain distance uses less fuel, regardless of how long it takes you to get there. Now the mileage gain may not be worth it to you if you have to suffer 3 more hours, but it doesn’t in any way cancel out your fuel savings.

I guess it depends on how much you value your time and how onerous or inconvenient the extra travel time would be. Unless your car is super-aerodynamic though, you’d certainly burn more total gas making the trip at 75 versus 55. Mileage (in the US) is miles per gallon, not hours per gallon.

That’s part of it, but wind resistance has the effect of requiring higher RPMs to maintain speed. So regardless of how aerodynamic your vehicle is, it comes down to how many RPMs you need to maintain your speed.

This is also why drafting works so well (see the Mythbusters episode). Less wind resistance means less RPMs, because your engine doesn’t have to work as hard to maintain speed. Less RPMs = less fuel use.

Sort of. First of all, the actual term is “superelevation”. An “embankment” is a slope that has been flattened by adding dirt (as opposed a “cutting”, which was flattened by removing dirt).

Anyway, the superelevation is determined by the “design speed”, which is the maximum safe speed of travel. For US interstates, minimum design speeds range from 55 to 75 mph, depending on the terrain.

If you want to start an exciting career in road design, you should check out this book.

This fifth edition of AASHTO’s “Green Book” contains the latest design practices in universal use as the standard for highway geometric design and has been updated to reflect the latest research on superelevation and side friction factors as presented in NCHRP Report 439. New exhibits in Chapter 3 will help designers to quickly and accurately determine the side friction factor used for horizontal curve design, the superelevation rates for various curve radii, and the minimum radii with normal crown for each of the five maximum superelevation rates.

On flat terrain with completely straight roads, wouldn’t the maximum design speed be higher than 75? Or is 75 considered the highest safe cruising speed? I thought cars on the Autobahn regularly travel faster than that.

They don’t. (as far as I know) I’m pretty sure they engineer roads such that they can support a maximum speed… a good example of this is the freeway exit just after the Caldecott tunnel in the Bay Area. (If you’ve ever used it, you know the one I mean… it’s a super sharp U-turn that then takes you down a hill & under the freeway. Max posted speed: 35 mph) However, you can take the curve as slowly as you want, and you won’t slip off the road.

The minimum speed limits that some freeways have are posted in order to keep traffic flowing, and possibly to avoid rear ends. The only roads that require a certain speed to stay on it are high-performance race tracks which were all the vogue in car commercials of the 70’s for some reason.

The 70’s: the decade that keeps on giving.

The ideal policy response to climate change and the oil mess is just increasing gas prices to factor in those externalities.

If the public wasn’t so insanely opposed to taxing energy usage, even revenue netural - notice what utter hell politicians catch every time gas prices go up - then yes, it’d be a bad policy response. They are, though, so I’m not sure what to do other than stuff like this.

65 in the great state of New Jersey

It’s 65 here in OH, too. Trucks have to go 55. It’s hard for me to get used to it. Apparently, enforcement is pretty good too. We have dedicated police vehicles for pulling over trucks and checking loads and such. In the three weeks I’ve been here, I’ve seen probably 3 trucks pulled over by these vehicles.

Maybe Robert has an oil well in his back yard so he could spend that 3 hours producing more oil than what he would have used going above 55 mph.

I don’t know what you mean by “maximum design speed”.

The design speed is the maximum safe travel speed. It is distinct from “operating speed”, which is the average speed observed in the absence of congestion. Clearly, operating speed should be lower than design speed.

All US interstates must have a design speed of at least 55 mph in urban and mountain terrain, 65 mph in rolling terrain, and 75 mph in flat rural terrain. I’m sure a lot of interstates exceed those specs, but I doubt the over-specs are regulated.

I’m not sure what you specifically mean by “a design speed of at least 55mph in mountain terrain” but I can point to several mountain passes on I90 where you are forced to slow down below 55 due to tight curves.

Ech. My Jeep does best around 65, not 55. Every mile counts right now, really, so I hate that idea.

Also, that will NEVER fly in Phoenix or Vegas. People routinely do 75+ on freeways, plenty of areas are 90MPH even at rush hour.