**Contributed by: evilscientist**

Now there are a few themes that go through the comments of people who complain about the police enforcing speeding laws. I've seen that there are four basic themes which I provide the following quotes from various commenters on various news sites and Facebook:

*When the police eradicate break ins, theft , gangs, and solve those murder cases, then they can go after the working tax paying 'criminal' in a hurry to go to or come home from work.*

Two more of the "arguments" appear to come from a complete ignorance of the laws of physics:

*Unfortunately, Governments rely on income from enforcing artificially low speed limits.*

*Ridiculous...how about seizing cars from ppl who drive to slow...they're a hazard or the really bad drivers...just another cash grab by this city & the police!!!*

[sarcasm] Yes, speed limits are just there to take money from people who speed by cash hungry governments [/sarcasm]. Take the tinfoil hats off for a while and let the sun in. Yes, governments set the speed limits, but the limits are based on the recommendations from traffic engineers who have determined, based on the type of road, where it's located, how busy it's likely to be, what the likely traffic distribution will be and a host of other factors, what a *safe* speed should be. To this end, a little physics.

Now in mechanics (the branch of physics which studies how things move) pretty much everything comes down to energy. In the case of a moving car, they energy we're concerned with is kinetic energy. The faster you're moving, the more kinetic energy you have. To stop you have to get rid of your kinetic energy. Normally this is done by applying the car's brakes which then convert the kinetic energy into sound and heat energy. Now the thing about kinetic energy is that the amount you have depends on how massive you are as well as how fast you are moving. Kinetic energy increases linearly with an increase in mass. That is if you double the moving mass (at the same velocity) you double the kinetic energy and if you triple the mass you triple the kinetic energy. Velocity (speed) has a much greater impact on kinetic energy than mass. Kinetic energy increases as the square of velocity. That is if you double the velocity four times the kinetic energy and if you triple the velocity you get nine times the kinetic energy. So a little change in speed can result in a large change in kinetic energy.

So what does this have to do with road safety? Well as I stated before, in order to stop your 1000kg engine of death, you need to get rid of its kinetic energy. The braking system does this by converting the energy to other forms, primary heat, but can only do this at a certain rate. The rate it does this conversion varies by how hard the brakes are applied, but for sake of argument we will assume that the breaks are applied to their fullest providing the maximum braking power. It is important to note that the maximum braking power is unrelated to the speed of the vehicle at the time. Your brakes are just as effective at 50 km/h as they are at 100 km/h. The fact that the car's brakes can only convert kinetic energy and thus slow the car at a constant rate is a large part of how long it takes to stop the car.

Now it makes sense that the faster you are going, the longer it takes to stop the car and hence the longer distance the car travels before it stops. Since the kinetic energy goes up with the square of the velocity, the distance it takes to stop also goes up with the square of the velocity. This means that the stopping distance at 100 km/h is not twice what it would be at 50 km/h, but *four times* the distance. Using this handy stopping distance calculator you can work out what this stopping distance is. So at 30 km/h the stopping distance of the average car, on dry asphalt, is about 5m. This increases to 14m at 50 km/h and to 56m at 100km/h. It is also interesting to note that the stopping distance at 110km/h is about 68m or a difference of almost the 50m stopping distance. This means on the highway, on a dry road, it will take over half a football field to stop your car *if you are going the speed limit*. If you are travelling at 120 km/h the distance increases to 80m and at 130 to 95m. So at highway speeds it takes an additional 25-30m for a car going "only" 20 km/h over the limit to stop compared to the cars around it, on clean dry roads. As an aside, if you are going 50 km/h over the limit (160 km/h) the stopping distance is about 144m, almost twice that of someone going the speed limit.

Now the figures above are on clean, dry asphalt. If it's raining the stopping distance increases by about 27%. So even on wet roads stopping distances are 30km/h-6m, 50km/h-18m, 100km/h-72m, 110km/h-87m, 120km/h-103m, 130km/h-120m and 160km/h-183m. Which is why you're supposed to slow down when the road is wet. It gets even worse if there is snow or ice. Snow alone increases stopping distance about 133% (that's right, more than doubles the stopping distance) and ice increases stopping distance a whopping 367% (3 and 2/3rd times the stopping distance). This means that in the winter on snow filled roads that moron in the 4x4 that thinks (s)he is invincible and that the rest of you who slowed down don't know how to drive who speeds past you at 120km/h will take 189m to stop if there's just snow on the road and 378m if the road is icy. Makes you wonder who really doesn't know how to drive.

Of course the distances given above are from the moment that maximum breaking effort is applied. There is further distance due to the reaction time of the driver. The average human reaction time is 0.21 seconds. That is from the moment that someone sees something they have to react to and the time they begin to react is about 0.21 seconds. If it takes an additional 0.3 seconds to move your foot from the gas to the brake, there's upwards of half a second before the maximum braking effort is even applied. In that time the car is still moving at whatever speed it was moving. This means that 30km/h the car moves over 4m from the time you see the kid jump out in front of you and before your foot is even on the brake. At highway speeds this distance increases to over 15m at 110 km/h and at 160 km/h nearly 23m. This has to be added to the distance the car travels while the brakes are applied.

The practical upshot of all this is that in order for you to see, react and stop the car before hitting a child in a playground zone means the child has to be over 9m away from your car at 30 km/h when you first see them. This increases to 21m at 50km/h. Which is why you shouldn't speed through playground zones. It's not some grand government conspiracy to extract money from you. It's to safeguard the lives of our children.

On the highway it means that if you are at the 110 km/h speed limit and something happens in the road ahead of you, you need that something to be at least 84m ahead of you in order to stop in time. If you're doing 130 it needs to be 114m ahead in order to avoid hitting it and at 160, 167m. Again not a grand government conspiracy to take money from less than law abiding citizens. Also these numbers are for *clean, dry roads* remember that these more than double in the wintertime. Which means that anyone speeding, especially in the wintertime is a complete and utter moron.

Now only partially related to speeding was this comment:

[police should ticket people]*incapable of driving closer than 30 feet to the person*[in front of them]

where the commenter seems to be advocating for tailgating. Now the Alberta Driving Handbook states that the safe following distance is 2 seconds behind the vehicle ahead of you (this is also the figure that the various civilian and military defensive driving courses I've been too have stated). Since the following distance will increase with speed under the 2 second rule, just how fast do you have to be going that 30 feet is the safe following distance? The answer is 20 km/h. That's right, if you tend to follow people 30 feet or less (about two car lengths) you either have to be going 20 km/h or be stopped at a traffic light. At 50 km/h the safe following distance, under the 2 second rule, is about 28m (about 91 feet). At 110 km/h this distance increases to 61m (200 feet). The astute amongst you will note that this is less than the stopping distances at these speeds. That is because the 2 second rule takes into account that it will take time for the guy in front of you to stop as well.

Physics wins again. All commenter who stated that you should tailgate the guy in front of you is showing is just how bad a driver the commenter is. Which is the key to most of the commenters on various threads on speeding. The people whining about "artificially low" speed limits and the such are really just trying to justify their own poor driving and bad driving habits. My only hope is that it's not me and my family that pays the price when their skill and car can't cash the cheques their mouths and lack of brains are making by blatently ignoring traffic safety laws.

In terms of the car seizures suggested I don't really have that much of a problem with it since I do my best not to speed which means I'm not likely to be caught speeding 50+km/h over the limit. There is something to the argument about due process and that needs to be worked out. However that being said I think that if you've been speeding that badly, you should lose the privilege of driving, permanently since you're not responsible enough to be behind the wheel.

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Evilness

http://www.evilscientist.ca/article.php?story=2012110315340040