Strictly Enforced Speed Limits May Adversely Affect Safety

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st13phil

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Some interesting research that suggests a driver's hazard perception capability is diminished when they're dedicating concentration to speed limit compliance.

It will be interesting to see what the next stage of the research finds.
 
A 20 mph speed limit was introduced where I live in order to make the roads safer. At first, drivers tried to stick to it (there were teams of volunteers with speed traps reporting transgressors to Hampshire Constabulary), now it's largely ignored and drivers look out for the speed traps instead (you can tell by the sudden application of brakes). At 20 mph you are travelling nearly 30 feet per second (29 feet 4 inches if you really want to know). If it takes 1 second to look down and register your speed, you've travelled almost 10 yards not looking where you're going. Because of the lower speed limit you tend to check your speed more frequently. How does that make the road safer?
 
I think part of the argument is that should one have an accident the consequences will be much less severe, especially if you hit a pedestrian....

But I agree the probability of having an accident may well increase because of the need to check so often.
 
A 20 mph speed limit was introduced where I live in order to make the roads safer. At first, drivers tried to stick to it (there were teams of volunteers with speed traps reporting transgressors to Hampshire Constabulary), now it's largely ignored and drivers look out for the speed traps instead (you can tell by the sudden application of brakes). At 20 mph you are travelling nearly 30 feet per second (29 feet 4 inches if you really want to know). If it takes 1 second to look down and register your speed, you've travelled almost 10 yards not looking where you're going. Because of the lower speed limit you tend to check your speed more frequently. How does that make the road safer?

Possibly not. What it definitely does is to make any collision with a pedestrian more survivable according to the medics who patch pedestrian accident victims back together. While perhaps placing more onus on the driver to obey the speed limit - time perhaps for car manufacturers to provide cruise control that operates at 20mph ?? it will also give pedestrians more time to take avoiding action.
 
I asked the council how many accidents there had been over the last 10 years...none.
 
I asked the council how many accidents there had been over the last 10 years...none.

So your contention is that in this particular case the 20mph limit should not have been applied. This doesn't of course reflect on the effectiveness of the policy elsewhere just its application in the particular case you cited. I can only suggest that the speed limit may have been imposed as a more global traffic calming strategy. If a series of routes are available to motorists they will inevitably gravitate to the line of least resistance. Imposing speed limits on nearby roads which may have a speeding problem would invariably drive motorists on to your road if it didn't have the same limit? the so called rat run syndrome. One possible explanation?
 
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For me, the biggest issue with speed limits is that the generally consistent correlation between road characteristics and the posted limit has largely disappeared.

This means that no longer can you be reasonably confident that if you select a speed that is appropriate to the characteristics of the environment then you can also be 95% certain that you are compliant with the legal limit. You therefore have to devote additional attention to scanning for speed limit signs and checking the position of the needle on the speedo instead of actually driving. This cannot be sensible.
 
Biggest reason in my mind to use cruise or limiter in these situations, then you can maintain more attention outside where it may be needed.
 
For me, the biggest issue with speed limits is that the generally consistent correlation between road characteristics and the posted limit has largely disappeared.

This means that no longer can you be reasonably confident that if you select a speed that is appropriate to the characteristics of the environment then you can also be 95% certain that you are compliant with the legal limit. You therefore have to devote additional attention to scanning for speed limit signs and checking the position of the needle on the speedo instead of actually driving. This cannot be sensible.

This touches on my remark vis a vis cruise control. According to some its lower effective limit is often 25mph. I have to confess to never having put this to the test in my relatively recent Merc. Maybe others can relate their experience on the lower limits of cruise control on their car ?
 
RonFleet said:
I asked the council how many accidents there had been over the last 10 years...none.
How many times have you been killed whilst driving over the last 10 years? If none, does that mean you don't need seat belts, airbags, crumple zones or any other safety devices?
 
whitenemesis said:
But I agree the probability of having an accident may well increase because of the need to check so often.
BUT, the 20mph limit hasn't replaced unlimited stretches of road. It's most likely replaced a 30 limit, which also needed to be checked. Surely people shouldn't need to regularly check their speed at short intervals. We're not talking about judging 70mph on a four-lane motorway here, it's 20mph on a narrow residential road with countless comparison points in peripheral vision to help maintain a steady speed.

Sounds like yet another excuse for people to ignore speed limits where the potential dangers (especially to pedestrians) are clear. The difference in time to drive through an area with 20mph limit compared to 30mph is likely only a matter of seconds. Are those seconds really more important than a child's life?
 
I can concentrate on looking for this...
bmw-amulett-i01.jpg


OR - I can be concentrating on looking at this...
electric-car-labs-2.jpg


It's hard to concentrate 100% on both.

Further - it's difficult to regulate speed by engine note (as advised by IAM) in an automatic, and to make things possibly worse, most cruise controls won't work at 19mph... but every moment looking at the speedo is a moment not looking at hazards.
 
Meldrew2 said:
I can concentrate on looking for this... OR - I can be concentrating on looking at this... It's hard to concentrate 100% on both. Further - it's difficult to regulate speed by engine note (as advised by IAM) in an automatic, and to make things possibly worse, most cruise controls won't work at 19mph... but every moment looking at the speedo is a moment not looking at hazards.
It's a limit, not a target with no tolerance. I'm not aware of any 20 limits on long stretches of clear road, so cruise control wouldn't be appropriate. A maximum speed of between 15 and 20mph within a zone gives up to a 25% tolerance that even the newest drivers should be able to maintain without frequent speedo checks.

As for the inability to concentrate on your speedo and kids emerging from behind parked cars, the trick is to only check your speedo when there's nothing interfering with your view. Also, at 20mph you have more time to spot the kids BEFORE they emerge.
 
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This touches on my remark vis a vis cruise control. According to some its lower effective limit is often 25mph. I have to confess to never having put this to the test in my relatively recent Merc. Maybe others can relate their experience on the lower limits of cruise control on their car ?
From personal experience, both Cruise and the Speed Limiter have a minimum operative speed of 20mph on the W204, W205, W212, R171 and R172.
 
Just checked the lower speed for cruise control and speedtronic in a small selection of Mercedes. It's 30kph = 18.6mph. So no more excuses.
 
From personal experience, both Cruise and the Speed Limiter have a minimum operative speed of 20mph on the W204, W205, W212, R171 and R172.


Thanks for that. I had a quick look at the W 205 online owners manual which stated a lower limit of 30kph which is just under 20 mph as you said. :thumb:
 
I'm not aware of any 20 limits on long stretches of clear road
While my OP was about the potential adverse effects of strict speed limit enforcement, not 20mph limits, as the conversation seems to have moved to the latter I'll have my two penn'orth.

If you want to see 20 limits on long stretches of wide, clear roads, visit Oxford where they've been deployed as a blanket - even on main arterial roads in and around the City. Compliance levels are pathetically poor, reflecting the irrelevance of the limit. Even TVP have publicly stated that they believe them to be a glorious irrelevance from a Road Safety perspective, and that they have much more beneficial tasks to perform than enforce them (cue mass indignation from the City Councillors). What they have achieved is that pedestrians and cyclists take even less care than they would otherwise when interacting with motorised traffic. So that's a raging success then :doh:

Funnily enough, in common with most experienced drivers I don't actually need a speed limit to be applied to a stretch of road to be able to select a safe speed at which to travel. The example of a narrow residential road with multiple reference points in peripheral vision is a case in point. It could have a NSL sign at the end of it, yet a competent driver would modulate their speed, probably not exceeding 20 or 25mph, and most likely travelling even more slowly.

There is a growing body of evidence that 20mph limits are resulting in more, not less, injury accidents. Unless and until the mechanisms at play are better understood, it seems to me that the dash for blanket 20mph zones is foolhardy to say the least.
 
Just checked the lower speed for cruise control and speedtronic in a small selection of Mercedes. It's 30kph = 18.6mph. So no more excuses.
Being pernickety, that only applies if you have set the instrument cluster to KM's. If it's set to MPH, the lower limit is 20mph.
 
st13phil said:
While my OP was about the potential adverse effects of strict speed limit enforcement, not 20mph limits, as the conversation seems to have moved to the latter I'll have my two penn'orth. If you want to see 20 limits on long stretches of wide, clear roads, visit Oxford where they've been deployed as a blanket - even on main arterial roads in and around the City. Compliance levels are pathetically poor, reflecting the irrelevance of the limit. Even TVP have publicly stated that they believe them to be a glorious irrelevance from a Road Safety perspective, and that they have much more beneficial tasks to perform than enforce them (cue mass indignation from the City Councillors). What they have achieved is that pedestrians and cyclists take even less care than they would otherwise when interacting with motorised traffic. So that's a raging success then :doh: Funnily enough, in common with most experienced drivers I don't actually need a speed limit to be applied to a stretch of road to be able to select a safe speed at which to travel. The example of a narrow residential road with multiple reference points in peripheral vision is a case in point. It could have a NSL sign at the end of it, yet a competent driver would modulate their speed, probably not exceeding 20 or 25mph, and most likely travelling even more slowly. There is a growing body of evidence that 20mph limits are resulting in more, not less, injury accidents. Unless and until the mechanisms at play are better understood, it seems to me that the dash for blanket 20mph zones is foolhardy to say the least.
Good points. I'd forgotten the crazy Oxford case. But at least it's always been crazy there!

You focused on competent drivers' ability to regulate their speed. It's a shame that those drivers appear to be very much in the minority.

I agree that a 20 limit does make people take less care. It's not just cyclists and pedestrians, it's motorists too. My house in in a 20 zone and it worries me the number of drivers I see operating their hand held phones. So perhaps we have to ask whether there's an optimum speed for overall safety? With no limit would everyone concentrate to the maximum because they know it's dangerous! But we have to consider the extent of damage/injury in the inevitable event of a collision, whether with another vehicle or a pedestrian. Thus we get back to what I believe is the main purpose of 20mph zones: to cut down on serious injuries. The intention isn't to cut down on incidents, as nice as that would be.
 

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