Why Nazi-like enforcement of speed limits is stupid. (1 Viewer)

[Whatever.]

When i'm driving down the road, in my girlfriends car, a ford fiesta, the weight of the car is 1520kg. If i'm doing 50km/hr, this means my kinetic energy is 14660 kJ.

Now, kinetic energy is reduced by the brakes at a constant rate, so the relationship between braking distance and speed is an exponential one.

So, using these relationships, this means a hummer, which weighs twice the amount of my fiesta, has kinetic energy of around 29000kJ.

Now, if we relate this to braking distance, this means the hummer would take about twice the amount of time to stop than my fiesta.

If we consider the speed i would have to be doing to have this sort of braking distance would be 70 km/h.

Would a policeman book the hummer for doing 50 km/h (as he is legally allowed to)? of course not!

Would I be booked for doing 60km/h on that street? Of course i would, even though my braking distance would be considerably lower than that of the hummer's at 50 km/h.

Another example. Say i was driving at 50 km/h along a road in heavy rain. When driving in the rain, your braking distance is doubled (as the RTA loves going on about). Would i be booked for doing 50km/h? Even though the braking distance would be equivalent to that of me doing 70km/h? Surely not. But in dry weather i would definitely be booked for doing 70.

Granted that the reaction time would be around 1 sec which means that by travelling at 50km/h would mean you travelled 6 m less before you started braking.

So how can the police be such nazi's on speeding when the difference between different cars and different weather conditions greatly exceed this small braking distance that the law enforcement police so closely?

 

[beatboxre]

When i'm driving down the road, in my girlfriends car, a ford fiesta, the weight of the car is 1520kg. If i'm doing 50km/hr, this means my kinetic energy is 14660 kJ.

Now, kinetic energy is reduced by the brakes at a constant rate, so the relationship between braking distance and speed is an exponential one.

So, using these relationships, this means a hummer, which weighs twice the amount of my fiesta, has kinetic energy of around 29000kJ.

Now, if we relate this to braking distance, this means the hummer would take about twice the amount of time to stop than my fiesta.

If we consider the speed i would have to be doing to have this sort of braking distance would be 70 km/h.

Would a policeman book the hummer for doing 50 km/h (as he is legally allowed to)? of course not!

Would I be booked for doing 60km/h on that street? Of course i would, even though my braking distance would be considerably lower than that of the hummer's at 50 km/h.

Another example. Say i was driving at 50 km/h along a road in heavy rain. When driving in the rain, your braking distance is doubled (as the RTA loves going on about). Would i be booked for doing 50km/h? Even though the braking distance would be equivalent to that of me doing 70km/h? Surely not. But in dry weather i would definitely be booked for doing 70.

Granted that the reaction time would be around 1 sec which means that by travelling at 50km/h would mean you travelled 6 m less before you started braking.

So how can the police be such nazi's on speeding when the difference between different cars and different weather conditions greatly exceed this small braking distance that the law enforcement police so closely?

Start a World War 3

 

[Graney]

M8 you are dumb if you don't know why

 

[lolokay]

nice physix m8

 

[ilikebeeef]

When i'm driving down the road, in my girlfriends car, a ford fiesta, the weight of the car is 1520kg. If i'm doing 50km/hr, this means my kinetic energy is 14660 kJ.

Now, kinetic energy is reduced by the brakes at a constant rate, so the relationship between braking distance and speed is an exponential one.

So, using these relationships, this means a hummer, which weighs twice the amount of my fiesta, has kinetic energy of around 29000kJ.

Now, if we relate this to braking distance, this means the hummer would take about twice the amount of time to stop than my fiesta.

If we consider the speed i would have to be doing to have this sort of braking distance would be 70 km/h.

Would a policeman book the hummer for doing 50 km/h (as he is legally allowed to)? of course not!

Would I be booked for doing 60km/h on that street? Of course i would, even though my braking distance would be considerably lower than that of the hummer's at 50 km/h.

Another example. Say i was driving at 50 km/h along a road in heavy rain. When driving in the rain, your braking distance is doubled (as the RTA loves going on about). Would i be booked for doing 50km/h? Even though the braking distance would be equivalent to that of me doing 70km/h? Surely not. But in dry weather i would definitely be booked for doing 70.

Granted that the reaction time would be around 1 sec which means that by travelling at 50km/h would mean you travelled 6 m less before you started braking.

So how can the police be such nazi's on speeding when the difference between different cars and different weather conditions greatly exceed this small braking distance that the law enforcement police so closely?

I doubt they're stuffed to check the age, model, weight, wheels, brakes, and general state of every car that went over the speed limit, not to mention the air humidity percentage, local weather and the angle of the road on which the car was driving on.

 

[beatboxre]

nice physix m8

University of BoS, Bachelor of Whatever. (Physics 101, with alot of data missing)

 

[Graney]

Hi iM a roads engineer

speed limits are max safe limits, based on average cars. Police cannot easily judge on the basis of a cars performance. Lots of performance cars may easily be able
to do faster and brake later etc, but the average car can't ( theycan, but speed limits are conservative)

 

[pman]

tire traction will make more difference in this day and age, the hummer will stop in a similar distance to you in the dry because it loses traction at the same point! The hummer may even stop quicker because it has more drag

 

[seremify007]

In hypothetically identical conditions yes the original poster raises a good point... but heavier cars also tend to have much better brakes and better tyres which reduce stopping distance anyway.

My car weighs 1.6 tonnes but I bet it can pull up a lot quicker than most other cars which are in it's price range and many cars which are lighter (not counting a Lotus).

At the end of the day, at most road legal speeds experienced by most drivers, it's driver reaction and knowledge on how to control the car which are more likely to have an impact on overall safety.

 

[SnowFox]

I would think to many variables in the equation would confuse anyone, so thats probably why, as graney so aptly put it, they use a maximum safety measure for a average car.


Besides, most cars which have stock high end engines usually have stock high end braking systems.

Still have yet to see a Lotus fly into a tree, but i see hell of a lot of bogan movers wrapped around them.

 

[Riet]

wow you have absolutely no grasp of physics. Heavy cars can and do regularly outbrake lighter cars. Braking distance is far more dependant on the tires than the brakes, on modern cars, because even economy cars have brakes strong enough to potentially lock the wheels. force due to friction is directly proportional to weight, and so is (negative) acceleration, so a heavier car wont necessarily stop slower, the car with worse tires will.

 

[zaxmacks]

lol kilojoules come from food not cars

who is this bourke?

 

[ilikebeeef]

lol kilojoules come from food not cars

who is this bourke?

Kilojoules is a unit for energy. Cars have energy when they move.

 

[zaxmacks]

Kilojoules is a unit for energy. Cars have energy when they move.

cars dont eat food, muff butt.

 

[murphyad]

Drive to the road conditions and all will be well.

 

[ilikebeeef]

cars dont eat food, muff butt.

Energy is not found only in food. It's everywhere. The energy that cars use comes from burning petrol.

 

[Graney]

the government should increase tyre grip standards

 

[chuboy]

Why? Plenty of accidents would be reduced if they didn't let bogans who think they are top shit drive around town all day in their illegally modified cars =/

You don't need brand new tyres to drive safely, you just need to not be a complete imbecile.

BTW just to expand on the OP, brakes actually don't reduce energy at a constant rate, the rate of deceleration is dependant on factors such as tyre temperature and pressure, and particularly the temperature of the brake pads. The harder and longer you brake, the worse the braking power.

 

[zombie omie jay]

Why? Plenty of accidents would be reduced if they didn't let bogans who think they are top shit drive around town all day in their illegally modified cars =/

You don't need brand new tyres to drive safely, you just need to not be a complete imbecile.

BTW just to expand on the OP, brakes actually don't reduce energy at a constant rate, the rate of deceleration is dependant on factors such as tyre temperature and pressure, and particularly the temperature of the brake pads. The harder and longer you brake, the worse the braking power.

1) oh ok accidents expert
2) brand new tyres help as a lot of cars have shit tyres/bald tyres

 

[SnowFox]

@#2- agreed.