Braking up a hill

[JamesC]

Wow - I love this place - just spent about an hour reading through the forums.

 

Anyway - my question relates to this: (cue violins)

 

 

Basically - someone was driving too quickly and went in to me as I pulled out. There is no way I could have seen him - but of course it's probably going to be considered my fault

 

Now, I used this peice of literature:

 

http://www.harristechnical.com/articles/skidmarks.pdf

 

to make this:

 

http://www.mintylamb.co.uk/crashomatic.php

 

14 metres of skidmarks means he was doing around 30mph - which is on the limit - and certainly too quick for the residential street.

 

HOWEVER - one big factor (I think) is the fact he was going up a hill when he went in to me - which I believe would further increase his initial speed (not to mention the fact that the skidmarks go on after he hit my car! - but thats another matter!)

 

Anyone want to guesstimate how much this would effect it? I have averaged the hill out at a 1:16 (for every 16 metres forwards it goes 1 metre up)

 

 

James

[Klaynos]

it depends alot on the gradient and mass of the car...

 

then it's mass * 9.81 * cos Angle

[swansont]

Yes, you are correct that going uphill will, in principle, decrease the braking distance, so that he must have been going faster.

 

1/2 mv² = Fd + mgh

 

Fd is the energy used up in the brakes due to the skid, so that's the equivalent of 1/2 m (30mph)² (F is the average braking force, d is the distance, 14 m)

 

m is the mass of the car, g is 9.81 m/s² and h is the vertical distance (height) change over the path.

 

So the mass cancels out, and h is about a meter. Now convert all the units and do the math

 

You get about 31.5 mph, asuming I did the math correctly. But all this ignores the energy lost in the collision with your car. The speed has to be even higher than that.

[JamesC]

Cheers guys... would you believe it - after posting that I happened to do a quick Google and found a page I never found before (having been looking all day):

 

https://atiam.train.army.mil/soldierPortal/atia/adlsc/view/public/7670-1/fm/19-25/CH12.htm#img062

 

So for sure he was doing 30+ - considering the damage done to my car - a fair bit I reckon! I don't think that's a 5 or 10mph nudge...

 

I guess now I have to work things backwards and see what speed he would have to be doing to be driving "safely" - if you work out the reaction times (using 1.4s as the time it takes to notice something and put your foot on the brake) - 20mph would mean he would have stopped several metres before he would have ever hit me.

[CPL.Luke]

wouldn't you also have to have a account of how far he had his foot down on the break and the friction in the road, average breaking distances for his car (from the car company might do something similar) in order to figure out how fast he was going?

[JamesC]

His foot is down on the brake enough to lock the brakes (and thus cause a skidmark) - so that's not an issue. None of the forumula I have found take in to account tyre size yet - but perhaps this is insignificant - as it seems is the weight of the car. Those two may be interlinked (bigger car = bigger tyres)

[CPL.Luke]

you should probably check the manufacture's website, because they should have the data your looking for, as in the mass of the car, tire size (if it matters) and most importantly somewhere they probably have data on the distance it takes the car to break in normal and other circumstances

 

at the very least you could request the information directly from the company, I'm sure they have it

[Klaynos]

Was it reported to the police because I know the police in the UK, which from your image I guess you are, used to be able to work it out quite accurately... (my uncle was a traffic cop)...

[JamesC]

I didn't report it at the time - I was a little shaken. I realise in hindsight now that I should have. Still - my amateur investigation will hopefully give something for the insurers to look in to. I still think its going to be blamed on me - but maybe I can get a 50/50 out of it.

[Douglas]

- if you work out the reaction times (using 1.4s as the time it takes to notice something and put your foot on the brake) - .

I think 1.4 s is excessively long. I think .75 s would be more accurate.

 

My reaction time, from the time I see..say....a red light, to the time I can flick a switch is around .24 s

[JamesC]

I found 1.4s as a time that is used in crash investigation. I've read anywhere from 1 to 2 seconds also.

 

Flicking a switch is slightly different to driving a car - you have to move your foot off of the throttle and on to the brake for a start... and that's not a straight forward movement. Also - sitting and waiting for a light and knowing its coming, and flicking a switch under controlled conditions isnt anywhere near the same as on the road. If you drove like that the whole time you would be a nervous wreck. I think people tend to relax slightly in the car - which is probably better for fatigue in the long run. You're also taking in a lot more visual information than looking for a light - there are all sorts of things to concentrate on.

 

However - interestingly - the car that crashed in to me was on hand controls! Not sure how that effects things.

[jdurg]

I didn't report it at the time - I was a little shaken. I realise in hindsight now that I should have. Still - my amateur investigation will hopefully give something for the insurers to look in to. I still think its going to be blamed on me - but maybe I can get a 50/50 out of it.

 

Sadly, insurance companies only really pay attention to the evidence recorded in a police report. Their reasoning is that the police are an independent party and are unlikely to alter the physical data in order to benefit one side or another. Also, the police reports are generated right there at the scene of the accident shortly after the accident took place, so there isn't really a whole lot of time for the evidence to be altered. While your investigation may give you peace of mind, I highly doubt it will alter the thinking of the insurance companies at all.

[JamesC]

I agree with this - at most I can hope to get 50/50 or 25/75 - however this is actually more for my own piece of mind - I feel some guilt having damaged a rare (well - will be rare soon) car.