20mph speed limits: what’s the data on pros and cons?

unforeseen

Apodemus wrote: »

Sadly, no! You missed that the x-axis scale is "arbitrary units" not mph.

So the safest speed is 20 arbitrary units per arbitrary time unit?

Norman_Castle

I suspect the 20mph zones have been introduced to try and bring speeds down to the current 30mph limits which are generally driven at 40+mph wherever possible.

wymondham

The stressy bit about driving with these limits is if they constantly change, which is distracting as you're always checking to see what the limit is now. I drive to the O2 from the M11 and that road in is 30/40/50/30 - all over the place making it a stressful trip even tough its simple. Add speed cameras in and it becomes a nightmare.....

brat

wymondham wrote: »

The stressy bit about driving with these limits is if they constantly change, which is distracting as you're always checking to see what the limit is now. I drive to the O2 from the M11 and that road in is 30/40/50/30 - all over the place making it a stressful trip even tough its simple. Add speed cameras in and it becomes a nightmare.....

I agree. It's not conducive to the safest driving. You're constantly checking your speed against the current applicable limit, and it quickly becomes the most important aspect of your drive, taking the lions share of your conscious attention.

It makes the 'compliant' motorist less safe than they otherwise would be.

SouthLondonUser

wymondham wrote: »

The stressy bit about driving with these limits is if they constantly change, which is distracting

Maybe it’s because I don’t drive/ride on motorways much, but I don’t find variable limits on motorways too bad, as they tend to be signalled quite clearly.
IMHO rules which change all the time over the course of a short London journey are way worse. A 5-mile journey can easily take you across 3 boroughs, which means 4 entities controlling the roads (3 boroughs + TFL), each with its own rules, from speed limits (eg 20mph in Lambeth and 30 in Westminster), to which vehicles can enter bus lanes and when. Bus lane signs are a huge annoyance because they are not always signed very clearly (often hidden behind trees or double-decker buses) and, even when they are, constantly checking whether the rules change after every junction (as they sometimes do) is a huge distraction. Car drivers are not affected as much, but motorcyclists need to pay lots of attention to be sure they can enter a bus lane.

SouthLondonUser

Mercdriver wrote: »

Seems to me that you need to ride more positively in the way some cyclists do so that others can't try and pass you in an unsafe way.

Not sure what you mean by positively. Care to elaborate?

If I am respecting the 20mph limit and other motorists overtake me aggressively and dangerously (eg too close), I am not sure how it can be my fault, and what I can possibly do, other than slowing down and letting these angry motorists overtake - which I try to do if I identify someone as potentially aggressive and dangerous, but it's not always possible to identify them on time.

If instead you are referring to cyclists undertaking me while I am turning left (and have clearly signaled my intention of doing so), well, first of all this behaviour is inexcusable (if they want to improve the gene pool they should do it without involving others...); then, well, again, not sure what I can do other than riding as close to the left as possible (which is the wrong way to go round a bend on a motorcycle, but safety first in a city full of idiots).

Mercdriver wrote: »

If you can't safely pass a slow cyclist, how wide is your bike? Is it a Gold Wing?

I am afraid you misunderstood, I was talking about speed, not width. If a cyclist rides at 15mph and the limit is 20mph, it isn't always possible to overtake him safely; maybe the road becomes narrower ahead (relevant for a car, less relevant for a motorbike), maybe there is a junction ahead with limited visibility, maybe the road is narrow, another vehicle is approaching in the opposite direction, etc etc. The end result is more potential for congestion as more and more vehicles are forced to drive at 15mph behind the bicycle, More congestion and more pollution for everyone. Not exactly the goal the 20is plenty campaign had in mind,

Mercdriver wrote: »

As far as traffic in central London is concerned, it still exists in the congestion charge zone.

Not sure I am following. Yes, of course it still exists. What I was trying to say is that I very much doubt that making driving in central London more expensive or more miserable than it already is would have a huge impact. It's unlikely to affect HGVs, delivery vans and minicabs. Minicabs might be regulated more tightly (maybe we have too many) but I don't think much can be done about the other categories. A huge city like London still needs these nasty ugly huge dangerous polluting vehicles to move goods around, very few of which can be moved in smaller and more eco-friendly vehicles.

Mercdriver

SouthLondonUser wrote: »

Not sure what you mean by positively. Care to elaborate?

If I am respecting the 20mph limit and other motorists overtake me aggressively and dangerously (eg too close), I am not sure how it can be my fault, and what I can possibly do, other than slowing down and letting these angry motorists overtake - which I try to do if I identify someone as potentially aggressive and dangerous, but it's not always possible to identify them on time.

If instead you are referring to cyclists undertaking me while I am turning left (and have clearly signaled my intention of doing so), well, first of all this behaviour is inexcusable (if they want to improve the gene pool they should do it without involving others...); then, well, again, not sure what I can do other than riding as close to the left as possible (which is the wrong way to go round a bend on a motorcycle, but safety first in a city full of idiots).


I am afraid you misunderstood, I was talking about speed, not width. If a cyclist rides at 15mph and the limit is 20mph, it isn't always possible to overtake him safely; maybe the road becomes narrower ahead (relevant for a car, less relevant for a motorbike), maybe there is a junction ahead with limited visibility, maybe the road is narrow, another vehicle is approaching in the opposite direction, etc etc. The end result is more potential for congestion as more and more vehicles are forced to drive at 15mph behind the bicycle, More congestion and more pollution for everyone. Not exactly the goal the 20is plenty campaign had in mind,


Not sure I am following. Yes, of course it still exists. What I was trying to say is that I very much doubt that making driving in central London more expensive or more miserable than it already is would have a huge impact. It's unlikely to affect HGVs, delivery vans and minicabs. Minicabs might be regulated more tightly (maybe we have too many) but I don't think much can be done about the other categories. A huge city like London still needs these nasty ugly huge dangerous polluting vehicles to move goods around, very few of which can be moved in smaller and more eco-friendly vehicles.

If you don't want to be overtaken where it's not safe follow the defensive lead of some cyclists and take control of the road by not riding to close to the edges.

Central London is congested enough in the daytime during charging times for me to ignore road transport altogether.

SouthLondonUser

Mercdriver wrote: »

If you don't want to be overtaken where it's not safe follow the defensive lead of some cyclists and take control of the road by not riding to close to the edges.

It seems like we are speaking two different languages!

I was talking about two issues:

1) Cyclists undertaking me while I'm turning left. I try to preempt this by staying as close to the left as possible when turning, so as not to leave room for idiot cyclists to undertake me. However, there's always the risk some genius thinks the half inch between my bike and the pavement is enough for him to squeeze through. The fact that a pushbike overtakes a powerful motorcycle should be food for thought: maybe I'm too prudent, or maybe the cyclist is a reckless idiot, or maybe both

2) Difficulty in respecting 20mph limits, especially on long and straight roads, because other motorists can (and do) overtake me dangerously. To be clear, on long and straight roads I tend to ride in the middle of the road, but angry motorists can still overtake me dangerously (eg by partially occupying the other lane).

Mercdriver wrote: »

Central London is congested enough in the daytime during charging times for me to ignore road transport altogether.

Not everyone has the luxury of choice, unfortunately. Like I said, I started commuting by motorcycle when the SouthernFail shambles made my commute a nightmare. Both last summer and this one, my line had 1 train per hour [sic] instead of the usual 1 train every 10 minutes during rush hour. Not to mention all the trains that were cancelled with no notice and no explanation.

NBLondon

wymondham wrote: »

I drive to the O2 from the M11 and that road in is 30/40/50/30 - all over the place making it a stressful trip even tough its simple. Add speed cameras in and it becomes a nightmare.....

I think you mean the A12? From where the M11 meets the North Circular round to the Blackwall Tunnel? I agree completely. Especially as it's an urban dual carriageway which could safely be 50 for most of the way.

brat wrote: »

I agree. It's not conducive to the safest driving. You're constantly checking your speed against the current applicable limit.

Even when you think you know the road... because it might have changed in the last month.

SouthLondonUser wrote: »

IMHO rules which change all the time over the course of a short London journey are way worse. A 5-mile journey can easily take you across 3 boroughs, which means 4 entities controlling the roads (3 boroughs + TFL), each with its own rules.

Wonder if that explains why the limits on the A12 change? Of course if I start looking for the borough boundaries as well as the limit changes - that will be even less safe.


As for the undertaking cyclists - there does seem to be a minority with a death-wish or perhaps a belief that they have divine protection... I used to have a driving commute that at one point went down a 1/2 mile hill. Traffic and the junction at the bottom meant that cars were in stop-start mode and never getting over about 13 mph. Cyclists would free-wheel down at 20-25mph. So some would pass on the left, some would go over the centre line and pass on the right (down the "wrong" side of the road) and some would switch between the two - passing at right angles across the nose of a moving car!

SouthLondonUser

Oh, and don’t get me started about the fact that there are downhill roads with 20mph limits, which I respect, but cyclists don’t. Or about the fact that, by law, cyclists are not technically required to respect speed limits other than in royal parks (oh, the hassle of fitting a £5 speedometer on a £ 1,000 pushbike…).

I genuinely do not know what the braking distance of a pushbike vs a modern motorcycle or car is, but I do know that even bicycles can be lethal (think of the cyclists with no front brake, who killed a lady while riding at ca. 20 mph), and I suspect that, in an emergency stop, it is much easier to lose control / jump over the handlebars of a pushbike than of a modern car or motorbike with proper tyres (I somehow imagine my Metzler Roadtec 01 or my Michelin Pilot Road 4 provide a slightly better grip than bicycle tyres), ABS, dual discs, etc.