On Fri, 28 Feb 2003 10:48:37 +0000, Paul Smith <
[email protected]> wrote:
> On Thu, 27 Feb 2003 19:33:25 +0000, "Just zis Guy, you know?" <
[email protected]> wrote:
>
>>>>Not many people are killed in traffic jams. Claiming that we could kill just as many people at
>>>>12 m.p.h. as, say, 60 m.p.h., _may_ be technically correct, but it is very misleading.
>
>>>Personally I find it enlightening.
>
>>In what way is it enlightening?
>
> Because in almost all cases most of the previous kinetic energy isn't released in the crash.
>
Paul,
Try a different calculation. (This scenario is basically one that was recounted to me by a friend at
school, I believe it actually happened but I can't be sure (one of the cars was actually a lorry).)
Two cars coming down a dual carriageway, one in each lane (overtaking)
Car comes up to a junction wanting to turn right and stops at the line, except that, instead of
stopping at the give way line stopped at the line dividing the two lanes.
Car on inside lane brakes hard but basically has nowhere to go.
The "real" accident was not fatal, remarkably. Aparently all the driver of the car that pulled out
could say while the firebrigade was cutting him out of the car was "I stopped at the line".
To a first approximation you can assume that delta-V for this collision is half the impact speed.
Work out the impact speed for a 50% risk of fatality. Now assume the original speed was 75mph (just
above the speedlimit) and work out where the driver on the dual carriageway first hit the brakes.
Now assume everything remains unchanged, the point at which braking begins is the same, the distance
to the collision is the same, reaction times are the same except that the car on the dual carriage
way was doing 70mph. Work out the impact speed, the delta-V and the risk of fatality.
I think you will be surprised.
In the "real" accident, the driver is probably alive because the lorry driver anticipated that he
wasn't going to stop and was going for the brakes before he even entered the dual carriageway but I
can't be sure. So yes, driver skill and anticipation do matter but, in this particular circumstance
I don't believe anybody could avoid this collision, the difference between a skillful driver and
less experienced driver is just the probability of injury or death and the original speed of the car
on the dual carriage way.
Of course, if we are going to allow cars to drive on dual carriageways at 70mph there are going to
be instances where even the very best driver can't avoid a fatal accident if someone else makes a
mistake. But society, though government, have decided that the "window of opportunity" for a fatal
accident is sufficiently narrow at 70mph that the benefits justify the risks. The problem with speed
is that the "window of opportunity" grows very fast for very small increases in speed.
There are probably some people who would argue that, in the sort of dual-carriageway I have
described where traffic can turn right across it the speed ought to be lower - and many of these
dual-carriage ways have had their speedlimits reduced to 50 or 60mph over the last few years of so.
Is that a reasonable response to the unlikely scenario I have described? Its hard to say. From a
cyclists POV then these sorts of dual carriageways are the worst, often you can't avoid them, the
nice B road you are cycling along joins the dual carriageway and then turns off again 50 yards
further along. 50mph traffic is MUCH preferable to 70mph traffic and 30mph traffic would be better
still but I'm not arguing that we should have 30mph limits just because a tiny proportion of
vehicles have real problems. This could be solved anyway. Building a dual carriageway across B roads
that walkers and cyclists may want to use ought to require alternative access, a bridge capable of
supporting people and horses is not unreasonable and I can't believe would add significantly to the
cost of building the road (OK, it would cost a lot to go back and fix these now)
And there are some people who say 70mph is no longer suitable, cars have better brakes, better tyres
etc and 80mph or more is reasonable. I don't agree with them - I'm one of the road users who can't
change the power or speeds I can acheive to any great degree. My car has a quoted top speed of
92mph, this gives me enough leaway that I can, if necessary, accelerate from the speedlimit on
motorways - not fast but it's enough for every circumstance I've ever been in. (Getting stranded in
the outside lane because there is no space to get back in and going faster and faster as the road
goes downhill is less enjoyable, as soon as the car goes much over 80mph (on the speedo) you start
noticing that it really isn't designed to be driven at these speeds.
But, if motorway speedlimits were raised to 90mph then I do have a choice, I can go out and buy a
more powerful car - my partners car will do speeds in excess of 130mph - it makes 70mph feel
comfortable with masses of headroom to go faster with little or no detectable difference in the
handling. As it is, I have problems in my car because a small but visible percentage of drivers are
going faster than I could possibly go and having them hairing down on me can make my driving
difficult and closing speeds here are probably around about 30mph. Imagine how it is for a cyclist
who needs to turn right on a dual carriageway where the closing speed might be in excess of 70mph
with these same drivers even for a fast cyclist
Speaking solely as cyclists (and most of us are motorists as well so we do have other hats to put
on) we couldn't care less what happens on motorways. But ... were the government to say "On the
motorway, while the speedlimit remains at 70mph, no speed camera may be set to trigger at less than
85mph - the police can still book but the cameras can't below 85" then the attitude that there is a
15mph "headroom" would also be transferred to other fast dual carriageways. Part of the reason many
of these roads can't be upgraded to M status is because there is no suitable alternative route to
non motorway traffic or A/B roads cross them and it would cost millions to build full bridges for
every little junction. The fact that they can't upgrade them because of these issues automatically
implies that there is likely to be some non motorway traffic using the road. For me, 40-50mph
traffic I can cope with OK. On almost any road I can reach 20mph for short distances, certainly none
of the hills on my daily commute prevent me doing that although I might not be able to manage it
from bottom to top on one of them and 30mph closing speeds is nothing more than I have to cope with
when approaching parked cars - i.e. its fast but normal cycling.
But try turning right (one of the staggered left/right crossroads across a fast dual carriage way.
You look behind - nothing you start moving out, you are now about three quarters across the first
lane. You check behind again, there is now a car about three hundred feet behind you - it is going
to take you 8 seconds to cross lane 2 and get onto the right filter lane, it is going to take the
car 3 seconds to get to you - what do you do? Any reasonable motorist would slow down and move back
into lane 1, anticipating "undertaking" the right indicating bike but there aren't any sane drivers
- I've had cars overtake me using the right turn filter lane - there's about 50 feet of tarmac to my
left including the hard shoulder. And don't forget that the looking behind you tends to cause you to
go right anyway so when someone is closing fast on you it's even more difficult.
It's frustrating for cyclists. Motorists want to go fast and yet we so often observe that their
average speed is no faster than our average speed. We so often get held up because a car that can
overtake will, even when there is a parked car ahead and oncoming traffic so it can't get through
while we had already seen the problem, the gap and that we didn't need to slow - yes maybe we should
have moved out earlier but sometimes the obstruction can be 10 seconds ahead - there might be a gap
in the oncoming traffic before we get there - then the car could overtake, and pass the parked car
without causing us any delay. So we try to be considerate but get penalised for it by stupid
motorists. (Of course, when you want the car to overtake and get to the lights quickly so they will
change they sit behind you so you have to stop as well
So perhaps now you will understand why you are unlikely to get much sympathy in u.r.c Speed in and
of itself is a problem to us, regardless of whether speedlimits are rigorously enforced or not. On
many roads we cycle on the speedlimit is irrelevant to average speed so people bleating about
getting done for 35 in a 30mph limit are sufficiently unobservant of all the delays in their
journeys that they actually think that the extra speed will get them somewhere quicker. And the vast
majority of the drivers who do overtake us don't do it well and maybe as much as 30% do it
dangerously in varying degrees.
Of course there are drivers who do do it well. And most of us have probably overtaken a cyclist
while driving and then realised that probably that wasn't the best time to do it. I hope this would
be the imperfect - overtake and then have to brake - rather than dangerous overtakes and I hope that
most of us realise when we have done it, rather than the majority of motorists who appear to be
blissfully unaware. You can't make people better unless you can make them realise they are doing
something wrong. They don't realise how selfish some of their manoeuvers are, and they don't realise
they are exceeding the speedlimit. - Lets allow everybody to carry video cameras and report bad and
dangerous driving. Bad driving maybe gets a letter in the post together with a video cassette[1] and
say 10GBP fine to cover costs. Dangerous driving gets 3points and a 60GBP fine. The fines going to
fund the people who will have to review the clips and ensure that they really are bad/dangerous and
whether the "injured" party did something to increase the danger or was genuinely calm and collected
during the incident. Indeed, some of the clips drivers would submit would probably indicate
dangerous driving on the part of the driver complaining as well. That would be an unpleasant shock
to some drivers "We have reviewed your clip and accept that the other party made a mistake and
pulled out too close. We have sent him a tape and fined him the 10GBP costs. However, your reaction
to the incident was unacceptable. Please present your licence together with this letter at a police
station of your choice within the next 14 days for an endorsement"
Regards,
Tim.
[1] some people like me don't have a TV, maybe a URL where you can view the clip on the web as well?
--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t," and there was light.
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