[Discussion] Traffic Laws

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A Meditation On The Speed Limit

That was a pretty incoherent "documentary". The striking thing for me was the orderliness of everyone running at the same speed. Far less risk in that situation than normally, for various reasons.

Counterpoint

When drivers are traveling at higher speeds, they require more time to react once they see changes in the road environment ahead. Once they engage the brakes, the distance required to stop the vehicle is directly related to the speed of the vehicle at the time of braking and the vehicle’s weight; higher speeds and heavier vehicles simply take longer to stop, so consequently these factors result in the increased probability of crashes. Speed also impacts the severity of a crash. The force involved in a crash is directly related to the speed at the time of a crash: “The energy release is proportional to the square of the impact speed,” according to the Transportation Research Board’s Special Report 254: Managing Speed – Review of Current Practice for Setting and Enforcing Speed Limits. These factors show how effective speed management using the Safe System Approach, defined as kinetic energy management, can contribute to reducing speeding-related serious injuries and fatalities.

IMAGE(https://highways.dot.gov/sites/fhwa.dot.gov/files/images/08_PR-Winter2022_Feature_Speed-Management5.jpg)

This graph shows distance traveled for perception/reaction and stopping by speed. Once a driver begins reacting, the vehicle travels a greater distance during an evasive maneuver, the driver has a reduced ability to steer around objects in the roadway, there is an increased risk that “an evasive steering maneuver will result in loss of control,” and more stopping distance is required.

Until vehicles are engineered for higher speeds, both in physical builds and assistive software and hardware, the tyranny of the square law will continue to dominate. Physics, reflected in driver's reaction times, vehicle stopping distances, force of impact and durability of the vehicles, will always trump perception. Go faster, hit harder, more likely to die.

Yes, differential speeds could cause accidents in situations where someone is, say, running at the speed limit, and another person is doing 30 over. But who is to blame for that speed gap? Traditionally, we blame the speeder. Is that a principle we really want to change, to recommend as policy that people speed up in an attempt to avoid accidents at a lower speed?

I mean... That's insane.

They need to shift a buncha cops from the nighttime trawling of minority residential hunting preserves and onto daytime highways. Just showing the flag can calm traffic a bit.

Robear wrote:

Yes, differential speeds could cause accidents in situations where someone is, say, running at the speed limit, and another person is doing 30 over. But who is to blame for that speed gap? Traditionally, we blame the speeder. Is that a principle we really want to change, to recommend as policy that people speed up in an attempt to avoid accidents at a lower speed?

I mean... That's insane.

My position is not that complicated.

There are more safe speeds and less safe speeds and those are determined not by the sign on the roadside, but by the road-design AND the state of the traffic on that road at that precise moment (and the weather, and the visibility, and the nature of your vehicle.....)

Sometimes you'll be safer speeding up to match the flow of traffic, instead of slavishly matching the speed limit. Sometimes you're safer being the slowpoke (e.g. wet/snowy roads).

That's it. It's risk management. That's all.

It's better to consider the speed limit as just one of those many variables than it is to use it as your leading indicator for how to drive safely.

Call it holistic defensive driving.

Robear wrote:

Until vehicles are engineered for higher speeds...

Some already are. My WRX will stop in a hell of a lot less tarmac than my wife's minivan will at the same speed. It has way more lateral grip. It'll perform much better in snow (cos AWD). It's literally engineered for higher speeds.

And that's comparing car to car, not comparing my WRX to an 18-wheeler, which is subject to the same speed limits.

As I've gotten older, more and more I've felt the safest speed to drive at is one where you don't feel the lateral g forces pushing you when taking a curve, which seems to be what speed limits aim for.

Chairman_Mao wrote:

As I've gotten older, more and more I've felt the safest speed to drive at is one where you don't feel the lateral g forces pushing you when taking a curve, which seems to be what speed limits aim for.

I used to be in the “speeding is fine” contingent when I was younger until I rolled a car and nearly died.

You know you've gotten old when you're bombing along the highway and suddenly think to yourself, "Wow, I'm going way too fast here; I need to slow down." Then you glance at the speedometer, and it's at 65mph.