20mph speed limit in residential areas to be made law (In Wales)

prowla

Stubod said:

Ibrahim5 said:

There is no point reducing limits without installing cameras. Motorists won't obey the laws of the road unless they are properly enforced.

..they could make it mandatory to fit all cars with GPs tracking systems that would either automatically restrict the speed to whatever the current limit is, or automatically fine you for any breach of the speed limit that you make. This would also help offset the cost of covid? 

I wouldn't trust the ability of a car to know the speed limit; there's a 30 limit residential road near me where my car thinks it's 80!

Bonniepurple

I was chatting to DH about this earlier.  There’s a stretch of road near where I work which goes from 60 to 30 for a very short stretch.  The roads leading off it are 30, 40 and 40.  The police are targeting it as there have been a spate of accidents.  Said accidents have included a police car on blue lights and plenty of rear end shunts as people brake suddenly.  Personally, I would say that the road should be 40 all along, with countdown markers from the 60 road to prevent sudden braking.

I have also noticed that when a road is NSL, people usually drive more to the conditions.  When that same road is reduced to 50, people seem to regard it as a target and drive at inappropriate speeds, leading to harsh braking (and therefore acceleration) and more accidents.

Barkin

MouldyOldDough said:

How many modern 6 speed manual cars can maintain 20mph in top gear?
Also remember "automatics" my 6 speed auto will be in 4th at that speed so emissions will be increased 

My 5 speed manual isn't comfortable at 20mph in 5th. It isn't comfortable at 30mph either. 
Drop down to 4th, and it's as happy at 20mph as at 30mph.

ISTM that some folk should really think about their driving style. 
20mph doesn't necessitate high rpm in a low gear!

facade

The simple answer is don't go to Wales, there are plenty of mountains and coastlines in England

ontheroad1970

It's not being done fro environmental issues, I don't know why people think it is

Bigphil1474

prowla said:

Stubod said:

Ibrahim5 said:

There is no point reducing limits without installing cameras. Motorists won't obey the laws of the road unless they are properly enforced.

..they could make it mandatory to fit all cars with GPs tracking systems that would either automatically restrict the speed to whatever the current limit is, or automatically fine you for any breach of the speed limit that you make. This would also help offset the cost of covid? 

I wouldn't trust the ability of a car to know the speed limit; there's a 30 limit residential road near me where my car thinks it's 80!

Where I work, the works vans have been fitted with GPS trackers, and excessive speed is picked up by management. They've had to limit the notifications to speed limit plus 50% to cut out the spurious notifications or drivers being picked up for exceeding the wrong speed limit. For example, as long as you are doing less than 60 in a 40 zone, you are okay. The technology doesn't seem to be there yet. Mainly seems to be the difficulty between the vehicle speedo, GPS speed, and location against road speed limit. There is a road I regularly drive down which Google Maps says changes from something like a 40 to 60 to 30 to 40, even though the signs say 40 constantly. I expect it will get there.

born_again

MouldyOldDough said:

How many modern 6 speed manual cars can maintain 20mph in top gear?
Also remember "automatics" my 6 speed auto will be in 4th at that speed so emissions will be increased 

I bet even @ tickover in 6th will be far more than 20mph.

Herzlos

Is a car in, say, 3rd gear at 20mph really using more fuel than one in 4th gear at 30? I'll need to try next time I'm out but I've never noticed a real difference in a modern engine where fuel consumption and engine speed aren't really related now.

Here's a response to an article from the BMJ saying that 20mph is more efficient and safer than 30mph:
https://www.bmj.com/content/368/bmj.m453/rr-0

It provides an interesting claim that accelerating from 0-30 uses 2.25x more fuel than accelerating from 0-20, and given that most 20/30 zones will have a lot more stop/start traffic than, say, a motorway, that's probably a significant figure. All the 20 zones I drive in have a lot of junctions and stopping so I'm never cruising at 20 for more than a couple hundred yards.

It also provides figures from City Of London research showing 8% lower NO emissions in diesels at 20 rather than 30.

Martyn1981

Herzlos said:

Is a car in, say, 3rd gear at 20mph really using more fuel than one in 4th gear at 30? I'll need to try next time I'm out but I've never noticed a real difference in a modern engine where fuel consumption and engine speed aren't really related now.

Here's a response to an article from the BMJ saying that 20mph is more efficient and safer than 30mph:
https://www.bmj.com/content/368/bmj.m453/rr-0

It provides an interesting claim that accelerating from 0-30 uses 2.25x more fuel than accelerating from 0-20, and given that most 20/30 zones will have a lot more stop/start traffic than, say, a motorway, that's probably a significant figure. All the 20 zones I drive in have a lot of junctions and stopping so I'm never cruising at 20 for more than a couple hundred yards.

It also provides figures from City Of London research showing 8% lower NO emissions in diesels at 20 rather than 30.

That's good to see, as it ties in with the numbers I posted on the other 20mph thread:

Edit - Just a thought, but in case anyone is interested I thought I'd check the kinetic energy, since velocity is more 'important', as the calculation is 1/2mv2, or half mass x velocity squared.

So, for a 1,000kg car (back then), it's kinetic energy at ~1mph would be about 100J, but at 25mph it would be 62,451J, or roughly 624x greater.

For comparison, at 20mph it would be ~40,000J, and at 30mph it would be ~90,000J.

90 being 2.25x 40.

So that 1.25x extra fuel is burnt to build up and store the extra kinetic energy in the vehicle at 30mph v's 20mph. But if you have to brake down to 20mph, then it's wasted in an ICEV, or only partially re-gened in a BEV.

Like you, I find these roads 'busy' with lots of interuptions, such as traffic joining, leaving, pedstrian crossings, and main traffic lights. I'm not sure it takes much longer (if at all) to drive at 20 v's 30.