Re: Advice about speeding ticket please

In message <qqkc13tv2rjqr2quli3p166g81b8qmsp6h@xxxxxxx>, nospam <nospam@xxxxxxxxxxxxxx> writes
Norman Wells <norman@xxxxxxxxxxxxxxxxx> wrote:

And when the same thing happens at 40mph, the limit will drop to
30mph....................and so on.

Which, of course, proves that it's nothing to do with road safety and
all to do with revenue generation.

No it doesn't. If you have a pinch point caused by road works,

So road works which don't create a 'pinch point' don't have reduced speed
limits?

The reason for reduced speed limits in such circumstances is usually safety. However, if congestion exists at some times of the day but not others, it may still be necessary to have a speed limit that applies all day regardless of whether there is actual congestion or not, since to do otherwise would be totally impractical.

it's easy to show that you in fact get more cars through the congested point
at 40mph than at 50mph, and more through at 30mph than at 40mph, due to
the fact that cars can safely travel much closer together at lower
speeds.

But it doesn't actually work like that does it. If all vehicles have a
strictly enforced speed limit then they can do nothing to reduce gaps
between them. All they can do is travel at the limit with existing gaps or
slow down which just swaps the gap behind them for the gap in front.

If congested traffic is entering the pinch point there shouldn't be many such gaps that need filling, so this is a bit of a red herring. In any case, it's likely to be inconsequential.

It's not a linear relationship, but down to the braking distance
being proportional to the square of the velocity. So, ignoring a
standard reaction time, cars travelling at twice the speed have to be
four times the distance apart.

Wrong.

No, right.

Assuming we are not concerned about large blocks of concrete or
something dropping from the sky a driver can brake and slow down more or
less as quickly as the traffic in front of him. The safe following distance
is all to do with reaction time not braking distances.

According to the Highway Code:

"105: Drive at a speed that will allow you to stop well within the distance you can see to be clear. You should leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance."

At 30mph the overall stopping distance is given as 75ft. At 60mph, it has risen over three times that to 240ft.

If drivers follow the 2 second rule (which is independent of speed) then
the flow rate of any lane of traffic is one per 2 seconds + the time the
vehicle takes to travel its own length. That time reduces as vehicle speed
increases without limit.

Indeed it does, and so too does the risk to life and limb. The 2 second 'rule' is actually ridiculously independent of the physics of moving masses which, if your education ever got that far, would have taught you that the energy possessed by that mass increases as the square of the velocity. And it's that energy that has to be dissipated by the brakes. As they can only dissipate a certain amount of energy per second, the braking distance increases in proportion to the square of the velocity.

As an example, I'm sure you wouldn't be too happy to be followed on a German autobahn by someone doing 150mph if he's just 2 seconds behind you.

The only other (and more significant) way flow rate can be increased is by
drivers sacrificing the 2 second rule, they may be more prepared to do that
at moderate speeds.

It depends what you mean by moderate speeds. At 40mph, the overall stopping distance is virtually the same as you would leave under the 2 second rule. However, that rule becomes increasingly reckless the higher the speed. At speeds under 40mph the overall stopping distance is less than the 2 second rule, so those applying it at such speeds are in fact holding everyone else up.

--
Norman Wells
NG

.

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