International Momentum Limit

Purpose of this page:

To raise awareness of the benefits of replacing the speed limits on public roads with momentum limits.

What is momentum?

To a physicist, linear momentum is mass times the vector of velocity of any object. When the object happens to be a car on the road, however, the only important component of the velocity vector is that component which is in the direction of propagation. The definition of momentum therefore may be simplified for this purpose to:

Momentum = Mass * Speed.

Qualitatively, momentum is the property of an object that causes it to resist changes of velocity (or, in this case, speed). It requires a relatively large force to slow down an object that has relatively large momentum, and a relatively small force to slow down an object that has relatively small momentum.

What is a Momentum Limit?

a Momentum Limit is a protocol wherein vehicles with large masses are forced, by law, to travel at slower speeds than vehicles with small masses, so that all of the vehicles on any given road are traveling with equal momentum.

Why is the momentum of vehicles on the road important?

Momentum kills. We have all heard that 'speed kills', but it does not. You are constantly involved in collisions with all sorts of things (like beta particles) that have a much greater speed than anything on the road. These things, despite their great speed, do not kill you because they have almost no momentum, owing to their small mass. On the other hand, when you are struck by something with sufficiently large momentum with respect to your own momentum, like an inordinately massive SUV (about 10^34 times more massive than a beta particle) striking a stationary pedestrian at a mere millionth of the speed of a beta particle, the transfer of momentum into the pedestrian's body causes his bones to be crushed and his internal organs to be ripped from one another, resulting in internal bleeding and death.

A 5000 pound SUV is, therefore, five times more deadly than a 1000 pound sports-car traveling at the same speed. A 5000 pound SUV traveling at a "safe" speed of 35 mph in a residential area is just as deadly as as 1000 pound sports-car speeding recklessly down the road at an insane 175mph!

Also, vehicles with greater mass and therefore greater momentum at a given speed take a longer time and distance to stop than vehicles with smaller mass (and therefore momentum). A 1000 pound sports-car traveling 35mph with a breaking force of 7500 Newtons will take about 7 meters and one second to stop, where as a a 5000 pound SUV traveling 35 mph with the same breaking force will require about 35 meters and 5 seconds! The SUV would have to be going 7mph to be able to stop in 7 meters and 1 second. (See below for equations).

Benefits of Momentum Limits instead of speed limits:

The main benefit of a momentum limit as opposed to a speed limit is, of course, safety. When a large vehicle strikes a small vehicle traveling at the same speed, more damage is done to the small vehicle and it's passengers due to the lower momentum of the smaller vehicle. This arrangement makes some people believe that large vehicles are 'safer'. Although the largest vehicle on the road may be the 'safest' for the one person who is driving it, it is the most dangerous for the billions of people who are not driving it. Furthermore, while drivers can increase their relative momentum (and therefore their perceived 'safety') by buying vehicles that are larger with respect to other vehicles, many objects on the road, like pedestrians or the median wall on the highway, can not change proportionally to reflect this trend. This means that even if everyone on the road were driving vehicles that were equally massive, larger vehicles would still be more dangerous (at the same speed) as they are more likely to kill pedestrians at low speeds and crash through median walls. Furthermore, the greater breaking distance and time adds greatly to the danger associated with vehicles that have more momentum.

Limiting the momentum of a vehicle would address all of these problems. Setting a Momentum limit on a road would, for instance make each car on that road equally likely to kill a pedestrian, should they happen to strike one. The momentum limit on residential roads could then be set to a value such that vehicles are not very likely at all to kill pedestrians, thereby reducing the number of senseless deaths. Notice that a speed limit cannot achieve the same thing. If all cars on the road are traveling with equal momentum, then when two cars collide, equal damage is done to each car (and their passengers), which is, by statistical definition, thee safest arrangement for everyone. Setting a momentum limit would also set the braking time and distance of all vehicles on that road to a constant value (given the same breaking force of each vehicle). We could then evaluate how much distance there is around, say, a blind curve and set the momentum limit around that curve such that any driver would have ample distance to stop should something unforeseen (like children playing, construction, cars pulling out of driveways) be happening there.

Instituting a momentum limit would also encourage people to buy smaller cars, which would have positive impacts on the environment, and save space (more on that later).

The part where I try to appeal to your emotions by giving vent to mine:

Many of us feel that we have never been given the chance to vote that it would be okay for some people to drive weapons around our neighborhoods, yet every time someone decides to try to one-up everybody else by purchasing a vehicle that is yet more massive than anyone else's, they are voting that it is okay for you to die. The rest of us, by implying our tacit consent are also voting that it is okay. If you did not intend to vote this way, then DO SOMETHING OTHER THAN IMPLY TACIT CONSENT! Write your statesmen, shout it from the rooftops, make angry web-pages, tell your friends.


Instituting a momentum limit would of course slow traffic to the slowest vehicle in any lane. Yes, this will suck for a while, but over time people will adapt by buying smaller vehicles and eventually the average speed on the roads will return to normal, but the safety of the roads will be greatly increased.


DeltaX = (DeltaV^2*M)/(2F)
DeltaT = (DeltaV*M)/F

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