Wednesday, 31 December 2008

Why speed limiters would be a poor solution

So once again at Christmas time the Government has dug out those old chestnuts for a new attempt at making some decent stuffing. This year it is sat nav driven speed limiters that make it onto the fire for a slow roast. These voluntary devices will save the world, cutting down on fuel emissions and reducing road accidents as we cut speeds to the legal limit we are told.

So the questions have to be are they a good idea and will they work?

Sadly there are a few technical issues to resolve.

1) The mapping is not available off the shelf. Yes, Ordnance Survey have a road network that underpins the geometry used by Teleatlas and Transport Direct/DoT and Navtec have their own alternative. But none have a legally accurate speed model. Nor is it as easy as mapping all the road signs as many don't match the legal orders to the metre.

2) If captured it will have to be updated for every vehicle every time a new road is added or speed limit changed else people wouldn't be able to drive on new roads. The current best commercial release is every 6 weeks from Ordnance Survey using an on-line push service to commercial customers.

3) GPS is accurate enough to work. Well most of the time it is, assuming the right chipset is used, the satellites are visible, your windscreen is not attenuating the signal, you have locked on, you are not in an urban canyon need I go on :-)

4) Even if you are 20m out it doesnt matter really. I suppose it doesn't.... if the road parallel has the same speed limit. My sat nave regularly places me on the service road parallel to the one I am driving in the Southampton area, and as readings are taken at 2sec intervals I have seen it on rare occasions jump off the motorway onto the feeder road alongside... oops that has a 30 limit... can I have my power back before... BANG too late... how much does a 2 hour closure of a motorway cost?

Then there are the safety issues

1 Overtaking the tractor on the rural road is far safer with foot down get by then brake back to the speed limit if you have exceeded it than being stuck at 60 edging past.

2 Will urban users really want to wait the 2 or more minutes it may take for the first satellite lock?

3 I use cruise control regularly on motorways and some straight urban roads when clear the problem with it is it doesn't stay exactly at speed its at best + or - 3mph over 10 minutes worse on steep hills, sure you watch the road not your speed readout but you relax too much

Finally will it work on fuel reduction?

I have to say from personal experience no. On the urban cycle congestion and the stop start acceleration will kill off your economy in any case so its on the open road that things will count.

But for the majority of law abiding users it will have no effect at all. In fact it will make matters worse!!

On what do I base this claim? Well every car I have driven overestimates speed at 70mph by between 4 and 8mph compared to a GPS at steady speed. Compare 3GPS at the same time and they agree to within .2 of each other. As a commercial driver this has given me the confidence that is safe (from a speeding ticket point of view) for me to drive to the GPS not the speedo. The result my long term economy has dropped from 57 to 54mpg (diesel).

I suspect that people like me, who generally drive within the limit, will be those most likely to fit the device. For us the thought would be if I put my foot down I still won't break the limit so I'll trust government technology to ensure I dont break the law.

Those who would make economy savings, the 85-90+ driver, wouldn't touch this with a bargepole in any case.

So is there value in what is proposed?

For the big picture, no this is a red herring.

However if the government are prepared to front the cash to enable local government to produce a nationally consistent set of speed limits in a digital form accurately reconciled with the legal orders AND the road signage, which is then freely available, regularly updated and integrated to all Sat-Navs systems on the market so that all a driver needs to do is glance at their sat-nav screen to know the speed limit then this particular chestnut might actually lead to something tasty and nutritious in time for Xmas 2009.


Anonymous said...

I reckon this is just another excuse for drivers who don't think properly about speed limits to think about them even less.

The speed limit on a particular stretch of road is defined as the maximum you ought to be doing - that doesn't make it a safe speed at all times.

Pete Roberts said...

Definitely agree anon in relation to government suggestions but surely a tool (my final paragraph) that tells you what the maximum speed is at any specific time is useful in ensuring you are keeping within that limit

Anonymous said...

"but surely a tool (my final paragraph) that tells you what the maximum speed is at any specific time is useful in ensuring you are keeping within that limit"

Totally unnecessary. No substitute for commonsense. And satnavs in themselves are potentially as dangerous a distraction from what's going on on the road around a driver as is using a mobile phone while driving.

Pete Roberts said...

Agreed in part common sense should prevail but the days of a logical speed limit have gone in a mix of 20,30 and 40 in urban and 50/national limits in rural areas. While I know all my local limits that is not the case in other parts of the country and a definitive reminder would be helpful.

On Sat navs if you fiddle I agree, however placed correctly and set right you can read the speed while still looking at the road ahead something that is not always true of the car speedo.