Safe Speed Forums

Picture the situation:

I live on a small cul-de-sac which leads out onto a fairly steep hill. At the junction of the cul-de-sac and the hill, looking left there is a straight road to the bottom. Looking right, you have about 100 yards of straight, then a sharp right turn.

People come over the hill at ridiculous speeds - I have had a number of occasions when I've pulled out from the cul-de-sac to turn right and been met by someone barrelling round the corner who's had to slam on the anchors. The consequences of being hit would be catastrophic, especially on a motorbike or even in a car as the driver side door would bare the full impact.

The speed limit is 30mph. There are hazard signs before the bend coming downhill, " Stay in low gear" signs, concealed junction signs and rumble strips in place, but people still ignore them.

How can such a hazard be controlled? The only thing I can reasonably think of is to implement a 20mph speed limit just for this section and have a single 20mph hump before the bend to "enforce" it.

Are you sure about the 100 yards to the bend? (And the bend is to your right when pulling out, correct?)

100 yards at 50mph takes just over 4 seconds.

Emergency braking distance of a modern car from 50mph is 49 yards (including 0.75 seconds thinking time) (NB: ON THE FLAT)

It's obviously important to get the facts right, but on the face of it it sounds pretty safe. ???

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
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Apologies, I was using a map and taking the distance to the beginning of the bend from the downhill side. The actual distance is about half that or maybe less from the cul-de-sac to the "vanishing point".

Ok. Fair enough. An even more accurate measure of the distance may be important or helpful.

Now, about the bend. What's the fastest 'safe speed' that vehicles can use on the bend? In the absence of other data (and it IS difficult) what's the fastest that you would drive round the bend?

It is possible to calculate max safe speed from a video of the bend if we also know the speed of the vehicle carrying the camera.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

In reasonable conditions I would say 25mph is the absolute maximum for a reasonable vehicle, significantly dropping when conditions are worse. It is also a notorious position for ice as water tends to reside in the surface cracks of the road and freezes quickly due to the unsheltered position. As I also mentioned the hill is very steep (steepest at the top where the bend is) and you need to be on the brakes otherwise you accelerate quite rapidly.

There are other hazards including parked cars down one side of the hill coming round the bend (residential - mainly semis and detached properties with no driveways) and a field on the other side which is a favoured cyclist / sledging run. There have been countless times where a football has come onto the road and the kids have run across to retrieve it, and I remember a sledge coming down the hill and onto the road.

The worst case of recklessness was when my ex-next door neighbour overtook a slower vehicle on the bend (I was behind) - fortunately nothing was coming the other way. It was frightening to watch and I'm pleased to say the chap has now moved away.

If we can determine the safe speed of the road, how can this be enforced? It's all very well putting up VAS but there are already plenty of warnings and they are ignored.

sounds like a case for active tyre spikes

If the police turned up on a regular basis and flagged down offenders, the spot would soon get a reputation for being a go-slow area.
However it WOULD require an investment in manpower, and with the way cameras have been used to abuse motorists it would not be without it's critics, unlike the past!!

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

OK. I'm getting a picture. Next question:

Are most folk driving too fast, or is it a minority of nutters (less than 10% say)?

If it's most then the environment misleads or catches out...

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Is there anyway of changing the priority so that the cul de sac pull out is a continuation into the road and the main road users would have to stop and give way to people coming out of the cul de sac? A give way sign would probably have a good slowing effect.

Paul, from my experience I'd say it's more than 10% but not a majority.

Teabelly, I like your idea in principle but I'd worry about someone flying over the hill top and meeting a car waiting at the give way line. After all they ignore all the warning signs that are already present.

There is a regular police presence as the local station isn't more than 200 yards further up the road!

It's a tough one. I think the single road hump might be the best way, if it can get the speed down just before the hill, you'd be at a safer speed for the hill and would have a better chance of stopping if you encountered someone pulling out (unless you blitzed over it!)

If we accept the 85 percentile rule, it's 15%

A couple from me:

1) Is there any visibility for drivers "in the know" coming round this bend toward you to have had a *previous* glimpse up into the cul-de-sac, possibly twixt other obstacles/bushes or similar?

2) Is the main road width such that a driver could be approaching the bend taking some of the wronmg side (being reasdy to get back in of course) to glean maximum mutual visibility and therefore buy himself an extra vital tenth of a second or so in the event of committed egress from the cul-de-sac?

Hi Roger,

a) Nothing, the road is obscured completely by immovable objects from all positions (i.e houses!)

b) You couldn't go on the wrong side of the road because the position of optimum visibility occurs immediately after the crest of the hill. It would be suicidal to move onto the wrong side at this point.

The problem can be compounded when a car parks (perfectly legally, but dangerously) close to the cul-de-sac which almost completely removes the view to the right from the cul-de-sac.

Cheers,
Martin

Nah. The curve seems pretty flat at the bottom from 80th to 90th percentiles. 85th is the lowest point, but the nutters are usually way above the 90th.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

If it's really more than 10%, I see three possibilities:

a) there's a local 'filter' that means you have more than the average proportion of nutters and/or

b) something in the environment misleads normally responsible drivers into false assessments and/or

c) your perception of the danger has become, err, over-heightened

I'm most interested in b) because it seems most likely and can be fixed.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

So why is (or rather was) the 85th percentile, and not the 90th percentile the chosen statistical value?

Because it's (believed to be) the lowest point on the curve.



From: http://www.safespeed.org.uk/speedlimits.html

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Thanks Paul.

As is my wont, my original post was somewhat flippant, but you can see what I was driving at.

I'm a bit confused - presumably if you're turning right then you're only in conflict with other traffic for about half a second, unless you're towing or something?

You have to travel carriageway width+vehicle length. Say 10+15ft = 25ft

At an average of 5mph (=7.3fps)(we're starting from zero, I presume) 25ft takes 3.4 seconds.

So I reckon 0.5 seconds is very light.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety