What is the true impact speed when two cars collide head-on at 50mph?

[Fredrik]

So to summarise (on a basic level):
Sitting in a car traveling at 100mph crashing into an immovable wall is much worse than sitting in a car crashing into another car also doing 50mph head-on.

If the situation is perfectly symmetrical (one car being the mirror image of the other and the collision being exactly head-on), there can't be any difference at all. In the real world, the situation is never perfectly symmetrical. If the collision isn't exactly head on, the driver won't accelerate as much, and is less likely to get crushed between the front and rear of his own car. Even the fact that the driver of the other car isn't sitting directly in front of him improves his situation somewhat.

 

[AJ_2010]

If the situation is perfectly symmetrical (one car being the mirror image of the other and the collision being exactly head-on), there can't be any difference at all. In the real world, the situation is never perfectly symmetrical. If the collision isn't exactly head on, the driver won't accelerate as much, and is less likely to get crushed between the front and rear of his own car. Even the fact that the driver of the other car isn't sitting directly in front of him improves his situation somewhat.

But we've already established that there is a difference.
A person in a car decelerating from 50mph to 0 is very much different than a person in a car decelerating from 100mph to 0 (in a crash situation).
The energy from hitting a stationary wall at 100mph is totally taken up by the car. Therefore, yes the crumple zone is larger, but a crumple zone is not linear in terms of force/deformation. And therefore deceleration increases dramatically in comparison to lower speed crashes.

With a head on, both traveling at 50mph, the crumple zone of BOTH cars absorb each speed of 50mph. Resulting in reduced acceleration at this lower speed. (Crumple zone smaller than at 100mph yes, but also less force required/involved which in turn equals less deceleration).

As the driver is connected to the car's deceleration in terms of seat-belt (and anything in front of him/her that makes contact ie.dashboard etc, the forces on the driver can be looked at as being relative to those that are on the car.

 

[Fredrik]

But we've already established that there is a difference.

I see now that you were comparing driving into a wall at 100 mph with a head-on collision between two cars both doing 50 mph. When I wrote the answer, I was somehow thinking of the car hitting the wall going 50 (the same speed as the two cars hitting each other). I guess I didn't think about what I was actually reading. Now that I am, I don't see why anyone would want to compare 100 mph into a wall with 50 mph head on. The relevant comparison is between 50 mph into a wall and 50 mph head on. My answer is correct for that comparison, but obviously incorrect if you double the speed of the car going into the wall.

 

[AJ_2010]

I don't see why anyone would want to compare 100 mph into a wall with 50 mph head on.

The police and the media seem to do it all the time.
This was one of the reasons I asked this question. Just to put my mind straight on the reasoning side of things.

Road safety is a big issue in the UK (and rightly so), but there is a constant pushing of big numbers relating to speed.
In my line of work I often use facts/figures/information from the accident database and speed is not ranked very high at all in terms of cause factor for injury accidents.

But most of our public tax money is aimed at reducing speeds on the road. And as such the media/police/politicians etc. always seem to push big numbers when referring to speed to cause 'shock' for use of a better word. (Well, this is my interpretation of events anyway).

Targeting speed is, as we all know, also a form of 'revenue' and generally the cheapest so called 'solution' to a 'black spot'.

Whether people agree with the above or not I'd just like to point out that its just personal opinion formed from work experience.

:)

 

[torquil]

Targeting speed is, as we all know, also a form of 'revenue' and generally the cheapest so called 'solution' to a 'black spot'.

According to the sources I've seen: for a given road, a lower speed limit leads to fewer deaths and less emissions.

Why? Less kinetic energy involved and less energy dissipation per unit distance.

 

[elemis]

According to the sources I've seen: for a given road, a lower speed limit leads to fewer deaths and less emissions.

Why? Less kinetic energy involved and less energy dissipation per unit distance.

However, it could be argued that since cars have a lower fuel efficiency at lower speeds it is likely that more HARMFUL Carbon Monoxide might be released.

 

[torquil]

However, it could be argued that since cars have a lower fuel efficiency at lower speeds it is likely that more HARMFUL Carbon Monoxide might be released.

If I can drive the same car at either 100km/h or 80km/h on different gears at the same engine RPM, it's pretty obvious which speed is more efficient, even per unit distance. This happens because everything is approximately the same within the car, apart from the much larger air resistance...

I think that considerations about engine efficiency vs RPM (which shouldn't cause very much difference anyway since cars have gear boxes), are second order effects that cannot "compete" with the unavoidable increase in energy dissipation caused by the rapidly increasing air resistance as speed is increased (unfortunately, it increases more rapidly than v to the first power...)

There is no question that a car with a gear box and a combustion engine, if constructed to do so, will be more efficient (less energy loss per unit distance) e.g. at 80km/h than at 100km/h.

Unfortunately, as interesting as this discussion is, I guess it belongs in a different thread.