What Nobody is Saying About a National 55 MPH Limit

Self | July 12, 2008 | Self

[supercat]

Running into a bridge abutment at 90, say, will demonstrate this.

Expressways are generally designed so that a vehicle that's on path to hit a bridge abutment will chew up a lot of guard rail first. I'm sure that isn't going to be a pretty scene at 90mph, or even at 55mph, but optimal deceleration from 90mph into a 100ft guard rail would be about 2.7g. I'm sure that a guard rail wouldn't provide optimal deceleration, so the vehicle occupant would feel significantly more than 2.7g, but I'm sure it would be a lot less than the 80g the person would feel decelerating in one meter.

[supercat]

If fuel is saved at a lower speed for autos, is fuel also saved if planes fly slower? If so, perhaps Congress should also edict a top speed for jet travel. /sarc.

FYI, planes have different speed limits and optimal speeds depending upon one's goals:

1. Structural speed limit--go too fast and the airframe will break apart (not all designs can reach this limit, even in a power dive)
2. Stable speed limit--go too fast and the plane may become uncontrollable (not applicable in all designs)
3. Level top speed--drag equals maximum thrust.
4. Optimum speed (fuel per mile)
5. Optimum speed (fuel per unit time)
6. Minimum level speed

It's worth noting that efficiency per mile drops off significantly below the first optimum speed; it drops off precipitously below the second. Some types of acrobatic planes could fly at 5mph, but they'd use enormous amounts of fuel to get anywhere at that speed.

[supercat]

And this method of pushing the states around IS constitutional.

Well, sort of. It's constitutional only when

1. Congress has the legitimate authority to offer the money in the first place (which it usually doesn't), and
2. The regulation in question will allow the spending to achieve a legitimate Constitutional objective more effectively than it would without it, and is narrowly tailored to fit that purpose.

If Congress wanted to pass a law stating that states which fail to enforce a 0.08BAC on specific roads funded by the Federal government would lose funding for such roads, that would probably be legitimate. There is no basis for the federal government funding all roads, however, nor is there a basis for the federal government demanding the enforcement of 0.08BAC on roads it does not fund.

[Gondring]

You're misunderstanding the point I'm making.

When traffic is faster, the braking distance is greater--and not in a linear manner. Therefore, if safe distances are to be maintained, you have to have LOTS more space between vehicles in fast traffic than in slow. Therefore, you get FEWER cars passing a point for every minute when cars are going fast than you do when they are going more slowly.

I know it sounds paradoxical, but look at the center (oversimplified) diagram...

or this PowerPoint presentation...

More can be found in Chapter 2 (2.3.2).

[supercat]

Therefore, you get FEWER cars passing a point for every minute when cars are going fast than you do when they are going more slowly.

I agree that the maximum safe traffic flux diminishes at higher speeds. Speed and traffic flux, however, are not independent variables. I would suggest that if traffic flux increases beyond the level that is sustainable at current speeds, but not beyond the optimum level, the increased flux will cause motorists to slow down. Motorists would not reduce the capacity of the road by traveling fast--rather, they would have their speed limited by road capacity.

It is certainly possible for people to reduce road capacity by traveling too slowly; further, for any non-trivial level of traffic flux there is a minimum safe average speed as well as a maximum. I'm not clear, however, how reducing the maximum allowable speed on a road would increase the traffic flux, except in cases where loading a right of way to full capacity would prevent other cars from entering.