How to make safety decisions - Wales urban 20mph

[IvanV]

Another benefit to drivers is that slower speed limits increase road capacity, and thus reduce variability in journey times for roads that are near capacity. This may well result in drivers being able to leave later to arrive on time for appointments, work etc., even if the average journey time has increased.

Not true, sadly.

It's true when the the base speed is 70mph. So reducing speed limits on motorways increases capacity. But it's not a monotonic relationship, it's a peaked relationship. It depends on the specific road and its features, but generally speaking on the open highway capacity is maximised around 45mph. But smoothing out stop-start waves is also important to improve capacity. I recently flicked through the post-opening project evaluation (POPE) for the the dynamic smart motorway around Luton. It increased road capacity by more than you'd expect, because stop-start fluctuations were reduced.

[IvanV]

What we also shouldn't do, though, is to look at a lot of very small numbers, and add them up without thinking about whether noise in the system means that each individual number is meaningless in the vast majority of cases...

What has happened here is that the average speed change was calculated, and used to deduce the total journey extension. I think as a bulk number, 1.0bn minutes is reasonably robust.

And, as I said earlier, if your experience is that your journey now takes longer, then it doesn't matter about the noise, you will have to plan on the basis that your journey now takes on average longer.

I don't think you can argue past the conclusion that this costs travellers in Wales about 1.0bn person minutes.

[IvanV]

You should probably also consider the benefits to drivers of driving slower. Firstly, generally it costs less per mile - in fuel, maintenance, and in insurance rates (to the extent that everyone is driving more slowly, and thus safely, or with lower repair costs in an accident).

Secondly, it's usually less stressful.

Thirdly, it gives more mental space for non-driving thoughts - I may well have a very valuable idea in the extra couple of minutes on my journey that's undertaken at lower mental load.

Not true sadly.

Well not true in terms of fuel consumption. Fuel consumption is minimised around 50-55mph. So it increases emissions too. I don't know what it does to maintenance. But you won't be turning the engine slower at 20 rather than 30, you'll just be driving in a lower gear and for longer. Might be a bit less tyre wear. My maintenance is mostly time rather than distance driven - and I think that's mostly true of typical car per year distances.

I don't find driving at 20mph low stress, I find it high stress. I find it hard to manage the car to maintain at that speed, the accelerator isn't very forgiving around there so my speed is never right. And I'm taking my eyes off the road to look at the speed all the time. The speedo isn't very accurate down there either. For that reason, there is actually more mental load.

[discovolante]

You should probably also consider the benefits to drivers of driving slower. Firstly, generally it costs less per mile - in fuel, maintenance, and in insurance rates (to the extent that everyone is driving more slowly, and thus safely, or with lower repair costs in an accident).

Secondly, it's usually less stressful.

Thirdly, it gives more mental space for non-driving thoughts - I may well have a very valuable idea in the extra couple of minutes on my journey that's undertaken at lower mental load.

Not true sadly.

Well not true in terms of fuel consumption. Fuel consumption is minimised around 50-55mph. So it increases emissions too. I don't know what it does to maintenance. But you won't be turning the engine slower at 20 rather than 30, you'll just be driving in a lower gear and for longer. Might be a bit less tyre wear. My maintenance is mostly time rather than distance driven - and I think that's mostly true of typical car per year distances.

I don't find driving at 20mph low stress, I find it high stress. I find it hard to manage the car to maintain at that speed, the accelerator isn't very forgiving around there so my speed is never right. And I'm taking my eyes off the road to look at the speed all the time. The speedo isn't very accurate down there either. For that reason, there is actually more mental load.

My 2017 van has cruise control down to about 19 mph, I don't know how many cars have that but I assume it's getting more common. Not always appropriate if you're on a road where you are having to pull in to let traffic pass or just generally speed up or slow down, but that's a situation where you probably wouldn't be accidentally speeding up too much anyway.

[dyqik]

What we also shouldn't do, though, is to look at a lot of very small numbers, and add them up without thinking about whether noise in the system means that each individual number is meaningless in the vast majority of cases...

What has happened here is that the average speed change was calculated, and used to deduce the total journey extension. I think as a bulk number, 1.0bn minutes is reasonably robust.

And, as I said earlier, if your experience is that your journey now takes longer, then it doesn't matter about the noise, you will have to plan on the basis that your journey now takes on average longer.

I don't think you can argue past the conclusion that this costs travellers in Wales about 1.0bn person minutes.

My argument is that most of those person minutes have zero value.

[dyqik]

Another benefit to drivers is that slower speed limits increase road capacity, and thus reduce variability in journey times for roads that are near capacity. This may well result in drivers being able to leave later to arrive on time for appointments, work etc., even if the average journey time has increased.

Not true, sadly.

It's true when the the base speed is 70mph. So reducing speed limits on motorways increases capacity. But it's not a monotonic relationship, it's a peaked relationship. It depends on the specific road and its features, but generally speaking on the open highway capacity is maximised around 45mph. But smoothing out stop-start waves is also important to improve capacity. I recently flicked through the post-opening project evaluation (POPE) for the the dynamic smart motorway around Luton. It increased road capacity by more than you'd expect, because stop-start fluctuations were reduced.

We aren't talking about open highway though. We are talking about urban streets, with frequent intersections, sideroads, parked cars, etc. There the peak capacity is likely at much lower speeds, because of the number of interactions between vehicles.

[nekomatic]

Fuel consumption is minimised around 50-55mph. So it increases emissions too.

This is categorically untrue as a general principle, and I’ll happily bet that it’s untrue of any model of car generally found on British roads.

[dyqik]

You should probably also consider the benefits to drivers of driving slower. Firstly, generally it costs less per mile - in fuel, maintenance, and in insurance rates (to the extent that everyone is driving more slowly, and thus safely, or with lower repair costs in an accident).

Secondly, it's usually less stressful.

Thirdly, it gives more mental space for non-driving thoughts - I may well have a very valuable idea in the extra couple of minutes on my journey that's undertaken at lower mental load.

Not true sadly.

Well not true in terms of fuel consumption. Fuel consumption is minimised around 50-55mph. So it increases emissions too. I don't know what it does to maintenance. But you won't be turning the engine slower at 20 rather than 30, you'll just be driving in a lower gear and for longer. Might be a bit less tyre wear. My maintenance is mostly time rather than distance driven - and I think that's mostly true of typical car per year distances.

Your figure here is for fuel consumption at constant speed. But fuel consumption in urban driving is largely driven by acceleration, with frequent stops and slowing. The lower speed you are accelerating to, the less fuel consumption in stop/start driving. Slower speeds also generally allow more gentle acceleration and braking, and may even eliminate some stops entirely, by allowing easier blending in and out of throttle. In particular, pulling out of side roads to merge with traffic is easier if that traffic is slower, and needs less net acceleration.

Maintenance on engines is driven by temperature cycles more than revolutions. A slower longer drive means better warm up and less time driving aggressively before the engine is warmed up, which reduces engine wear significantly. Maintenance on brakes, tires and suspension is speed dependent, as the loads increase significantly with speed.

I don't find driving at 20mph low stress, I find it high stress. I find it hard to manage the car to maintain at that speed, the accelerator isn't very forgiving around there so my speed is never right. And I'm taking my eyes off the road to look at the speed all the time. The speedo isn't very accurate down there either. For that reason, there is actually more mental load.

You have more time to react to traffic, pedestrians, situation. It's generally lower stress, if you don't try and drive exactly at the speed limit.

[dyqik]

By the way, I'm driving in the US, where standard urban speed limits are 25 mph, with lots of 20 mph, very occasional 30 mph, and decent amount of 35 mph zones. There's nothing magic about 30 mph speed limits in car design, or driving. And there's no need for speed limits to be either 20 or 30 mph.

Almost every town here has signs at the boundary saying "25mph speed limit, unless otherwise indicated". Large amounts of Cambridge are 20 mph limit, as is the downtown area of my small town. This massively aids pedestrian walking and cycling, which in turn reduces traffic.

[jimbob]

Fuel consumption is minimised around 50-55mph. So it increases emissions too.

This is categorically untrue as a general principle, and I’ll happily bet that it’s untrue of any model of car generally found on British roads.

Indeed. Air resistance is hugely important.

[dyqik]

Fuel consumption is minimised around 50-55mph. So it increases emissions too.

This is categorically untrue as a general principle, and I’ll happily bet that it’s untrue of any model of car generally found on British roads.

Indeed. Air resistance is hugely important.

In urban driving it's far more about acceleration and deceleration and overconsumption due to aggressive stopping and starting. Speed limits as low as 20 km/h (12 mph) are more efficient once there's enough traffic that you have to respond to it.

https://www.ifpenergiesnouvelles.com/ar ... affic-flow

(this study is for variable limits, used to manage speed and capacity, but the general point about overconsumption due to stopping and starting remains.)

Another study based on fixed limits is here: The peak efficiency occurs with limits around 15 mph. Note that average speeds are significantly lower than the limits, due to stopping and starting, etc. The modelled average speed with a 15mph limit (7.-7.5 mph, the same as TfL quote for London traffic) is 1 mph lower than the average (8-8.5 mph) with a 30 mph limit. And that can be entirely negated by people driving shorter vehicles.

[IvanV]

My argument is that most of those person minutes have zero value.

What's this? Those few coins in your pocket have no value, because it's only a few, there's not much you can do with that few, and you've got lots of other money when you want to do something serious.

Clearly the world doesn't work like that. The $1.50 in your pocket, or whatever, is still worth precisely $1.50, because money is fungible. Time is similarly fungible. Extensive research establishes that people will pay money to shorten journey times. In fact we have a very good idea just what it is worth to people, on average, and for different subsectors of society, and for different journey purposes, because so much research is done to establish those values. So actually that time is valuable to them. And the "because it is only a small amount, it's worthless" argument is a fallacy.

[lpm]

Time is not fungible.

Most people need a downtime in each day which is neither a work activity nor a leisure activity. Chillin'. The brain switched off from max speed. Staring out the window, idly scrolling Facebook, driving. There are minutes that cannot be transferred to work or childcare or social interaction.

[bolo]

Almost every town here has signs at the boundary saying "25mph speed limit, unless otherwise indicated".

Where you are, perhaps, but not in the US generally. I've lived in the US for 50 years now, and I can't recall ever seeing that sign. Not that the specifics of signage make any difference either way to the rest of this discussion.

FWIW, I'm with Ivan that "it's small, so it can't matter" is a bad argument. It's the same as the common argument that "it will cost you less than your daily cup of coffee" to make an annual cost of several $100 seem irrelevant. Or in my world, that "the government can clearly afford it, it's only a few $millions out of the annual federal budget of $squillions" which is true but not helpful when thousands of people are saying the same about other things that are also only a few $millions.

[nekomatic]

The $1.50 in your pocket, or whatever, is still worth precisely $1.50, because money is fungible. Time is similarly fungible.

This is clearly not universally the case. If I save 20p a day by buying a less expensive sandwich at lunchtime, at the end of the week I can spend a quid on a chocolate bar. If I save two minutes a day on my commute, I can’t spend ten minutes extra watching telly on Sunday. There is much less scope for moving time around like that, even if you’re extremely organised and have some sort of task sitting there that is essential to do but that you can do in two minute increments.

FWIW, I'm with Ivan that "it's small, so it can't matter" is a bad argument.

In general, I agree. In some specific cases, like this one, maybe not.

[Sciolus]

I'm rather puzzled that Ivan started this thread without thinking to type "Wales 20 mph regulatory impact assessment" into his favourite search engine. He could have saved himself an envelope.

The RIA makes clear that, although there are large uncertainties because nothing like this has been done in the UK before, the monetised cost of additional journey times (central £6.4b over 30 years) far exceeds the monetised benefits. But the measure supports a wide range of policies (Section 2.2), with the particular aims to:

2.2.4.1 Reduce injuries on the road network.
2.2.4.2 Encourage a change in travel behaviour, with people feeling confident, safe, and secure enough to increase their use of active travel modes
2.2.4.3 Improve the environment and economy of local communities by reducing the negative externalities associated with vehicle use.

Regarding the matter of whether lots of individual minutes can be added up linearly to get a sensible large number, it says:

It should also be noted that this dis-benefit accrues from the application of standard
constant values of time over which there is active professional debate 80 . There has
also long been academic debate around the application to small time savings 81
where the benefit of small time savings may be perceived to be small or negligible
relative to larger time savings. This being said current TAG guidance stipulates the
use of a constant value of time regardless of the sign or size of the time saving (see
further discussion in section 6.5).

[dyqik]

Almost every town here has signs at the boundary saying "25mph speed limit, unless otherwise indicated".

Where you are, perhaps, but not in the US generally. I've lived in the US for 50 years now, and I can't recall ever seeing that sign. Not that the specifics of signage make any difference either way to the rest of this discussion.

FWIW, I'm with Ivan that "it's small, so it can't matter" is a bad argument. It's the same as the common argument that "it will cost you less than your daily cup of coffee" to make an annual cost of several $100 seem irrelevant. Or in my world, that "the government can clearly afford it, it's only a few $millions out of the annual federal budget of $squillions" which is true but not helpful when thousands of people are saying the same about other things that are also only a few $millions.

This doesn't even work with money in the real world. It's really not worth the effort to pick up every penny you see lying on on the ground, and then take them to a bank to deposit in your account, unless you are near destitute.

Similarly, 5¢ deposits on returnable cans aren't worth the effort of collecting them and driving them to the appropriate place to return them to the majority of people.

Pennies in a bank account that you don't have to spend any time collecting are different to pennies that need to be deposited.

Neither "everything matters" or "nothing small matters" works perfectly in the real world.

[bolo]

I'll stipulate that almost nothing works perfectly in the real world.

IMHO there is a difference between "I personally can't be bothered to pick up that penny" and "dropped pennies may not be picked up".

Re can deposits, you surprise me. I haven't studied this, but a cursory google suggests they are actually quite effective. See e.g. https://en.m.wikipedia.org/wiki/Contain ... ted_States

[dyqik]

I'll stipulate that almost nothing works perfectly in the real world.

IMHO there is a difference between "I personally can't be bothered to pick up that penny" and "dropped pennies may not be picked up".

Re can deposits, you surprise me. I haven't studied this, but a cursory google suggests they are actually quite effective. See e.g. https://en.m.wikipedia.org/wiki/Contain ... ted_States

In my experience, around Boston, cans are generally picked up from recycling bins by the homeless, and returned by them.

[dyqik]

The overall point is that saving a couple of minutes a day in journeys doesn't have any significant value to most people, and you can't add up insignificant value to lots of people to get significant value - because it's always distributed.

Since we are talking about speed limits in urban environment, I'd be surprised if the difference was more than a few minutes on almost all journeys - longer journeys will start in an urban environment, drive to faster roads, and then only end in an urban environment again. The speed limit changes only affect the start and end of most longer journeys, so the delays don't scale with journey length.

Also, for journeys that are undertaken in order to arrive somewhere at a specific time, the average journey time also isn't so important as some upper percentile.

You can't just assign a fixed value per minute to time saved when it only arrives in small increments.

[bob sterman]

The overall point is that saving a couple of minutes a day in journeys doesn't have any significant value to most people, and you can't add up insignificant value to lots of people to get significant value - because it's always distributed.

If a couple of minutes a day don't have any significant value to you - would you mind doing a couple of minutes a day of work for me? Unpaid of course. Yes you'll be doing the equivalent of approximately 2 days work for me over a year (about 12 hours). But that doesn't have any significant value right? I could make a bundle if I could get lots of people to do the same!

Or perhaps, if you have a car, you wouldn't mind if I switched it for one with a dodgy engine that takes a couple of minutes to start each morning? Again - 12 hours a year you'd be sitting their groaning because it wont start - but that time has no value right?

[nekomatic]

I'm rather puzzled that Ivan started this thread without thinking to type "Wales 20 mph regulatory impact assessment" into his favourite search engine.

Thank you for doing so and sharing the informative outcome!

I also think Ivan has been lucky to so far escape comment on ‘I can’t drive safely at 20 mph because I have to keep looking at my speedo’.

[nekomatic]

If a couple of minutes a day don't have any significant value to you - would you mind doing a couple of minutes a day of work for me? Unpaid of course. Yes you'll be doing the equivalent of approximately 2 days work for me over a year (about 12 hours). But that doesn't have any significant value right?

And yet, if I somehow magically relieved you of two minutes of unwanted work every day, what would you have to show me at the end of the year? You’d definitely have knitted a sweater, or built a couple of bookcases, or done a quick online course in something, right? At minimum you’d have read a couple of extra novels. Surely.

It’s almost as if the value of and our perception of small amounts of time were only loosely correlated and highly context dependent.

[Fishnut]

I'm just amazed at all these people who are able to travel to such tight schedules that even a minute's difference will be an inconvenience. Traffic lights are always on their side, a parking space is always available right where they want it, nothing unexpected happens that means their journey ever fluctuates and makes having a buffer of a couple of minutes necessary.

[Sciolus]

If a couple of minutes a day don't have any significant value to you - would you mind doing a couple of minutes a day of work for me? Unpaid of course. Yes you'll be doing the equivalent of approximately 2 days work for me over a year (about 12 hours). But that doesn't have any significant value right? I could make a bundle if I could get lots of people to do the same!

Turn that around: How much would you be willing to pay me to do 2 minutes' work a day? Now, how much would you be willing to pay me to do 2 days' work per year? You would get far more value from the latter: under the former scheme, the kettle would barely have boiled before I knock off.

Let me quote the RIA again:

Another important caveat to this estimate is the nature of the journey time impact,
as individual journeys will on average only be affected by one minute and most
journeys affected by less than 2 minutes. When time savings are likely to be small
there is greater uncertainty about the opportunity cost of that time. The valuation of
small time savings has long been an area of debate. A discussion of this is included
in a review of the value of time by ITS Leeds in 2010 82 for DfT, including studies that
found the value of small time saving to be smaller or even zero for less than 5
minutes. Nevertheless UK and international practice calls for the use a constant
value of time when appraising journey time savings. Reviews 83 including the ITS
2010 review have tended to find insufficiently robust empirical evidence to formally
adjust values for small time savings and recommend the application of one constant
value.

And these references for you to go off and read:
82 See page 76 of:
https://assets.publishing.service.gov.u ... ft-006.pdf
83 See: https://assets.publishing.service.gov.u ... t-2015.pdf, Batley, R., Bates, J., Bliemer, M. et al. New appraisal values of travel time saving
and reliability in Great Britain. Transportation 46, 583–621 (2019). https://doi.org/10.1007/s11116-017-9798-7