Speed limit arithmetic

[igenlode]

"It took an average of three minutes and 38 seconds to drive one kilometre (0.6 miles) in the centre of London last year... This was partly blamed on widespread 20mph speed limits" -- *what*?

20mph is 32km/h (using the rule-of-thumb conversion ratio of 5:8). So if you were travelling at 32 kilometres per hour it would take (60/32)=1.875 minutes to cover one kilometre: 1 minute 52 seconds. Clearly the majority of the time spent driving in central London is already spent travelling at speeds far *lower* than 20mph; probably spent stationary in traffic jams consisting of other cars. So just how fast are they expecting drivers to sprint between traffic jams in order that increasing the maximum speed limit will reduce journey times?

Comments

[pangolin20]

Hmmm, I see the figure given for driving a kilometre works out to 16,514 km/h (10,262 mph), so I think I can work out how long they'd have to be stationary for a speed limit of more than 20 mph to make sense. The equation would be 0 * x + 20 * (1-x) = 10,262, which works out to 20 - 20x = 10,262, which gives us x = 0,4869. That means that drivers would have to stand still for more than 48,69% of the time for a higher speed limit to make sense. I can't tell if that's a sensible number or not (though it sounds rather high to me), but that might make it a bit clearer for you.

[igenlode]

In real life, of course, the problem is more akin to Achilles and the tortoise: the faster you accelerate away from the traffic lights, the sooner you hit the end of the *next* stationary queue of cars, and it simply isn't possible, even travelling infinitely fast between traffic jams, to reduce the overall journey time by increasing the speed limit.
(Which is why the authorities tend to adopt the counter-intuitive measure of *reducing* the speed limit so that the traffic spends more time making steady progress and less time accelerating pointlessly between forced halts. Given that the average speed of travel in London was famously measured at 7.4mph a few years back ("Most car journeys in the city would be faster if you ran"), if traffic is now doing as much as 10mph after the introduction of 20mph zones this would appear to be an improvement :-)

[pangolin20]

It seems that I was a bit too focussed on the maths aspect of the question, then...

As for your comment, I quite agree! I'd only add that the faster people drive between halts, the more abruptly they need to stop at the next jam, which could well cause a traffic jam of itself (as the speeders behind the braking car need to brake hard themselves). So I can see why making traffic go slower would make it go faster (and make it nicer to drive in), and it's good to see that it has apparently helped in London. And thanks for the article; it looks quite interesting!

[igenlode]

I initially read the opening line of that strip cartoon as "There's a certain type of brain that is easily distracted" -- which is probably a fairer way to look at it!

(My brain is word-focused and tends to get distracted by spelling mistakes and confusing punctuation... or by chasing after interesting historical roots and relationships between words and between languages :-)

And also by ridiculously tenuous fandom-adjacent connections, of course... I've ended up interested by the stormy career and relationships with his subordinates and with the authorities of a Soviet avant-garde theatre director -- despite the fact that I don't even like what I've seen of his work! -- via the fact that one of said subordinates (and acolytes; it was a fairly *intense* mileu) once spent a few months effectively moonlighting from his 'real' theatre career in order to make a film that I very much enjoyed... but continues to bring up the subject of his beloved director and their theatre work at frequent intervals, so that while pursuing the fandom of said film I was perforce exposed to quite a lot of information on this very tangentially related subject ;-D


The traffic article is really only saying the things that any cyclist, or anyone else who travels under their own power rather than by the mere depression of a foot pedal, had always known: that you use far more energy in constant start-stop travel than you do in continuing to travel at a slow speed, which is why jerking forward and then braking is physically hard work. And that cars which race ahead towards the next red light don't actually gain anything in the long run.

Even when I was a child in London we used to play race-the-traffic-jam; you weren't allowed to break out of a walk, although you could walk as fast as your little legs could carry you, and you used to mark a specific car in the traffic beside you when you joined the main road and attempt to arrive at the level crossing on foot before that car got there. About fifty per cent of the time, the child would win...

[pangolin20]

I initially read the opening line of that strip cartoon as "There's a certain type of brain that is easily distracted" -- which is probably a fairer way to look at it!

Being so easily distracted is why I bothered to comment on this post in the first place, along with having talked to you once before, and I did get some useful data out of my calculations, so that's indeed a fairer way to look at it (though the person saying that is of course not meant to be fully in the right).

(Since you mention confusing punctuation, I think I can note that I'm distracted by the absence of a closing bracket after your emoji, not that I mind, of course. And I'm generally quite interested in language, so I also do some of that chasing down myself.)

That certainly sounds like fun! Could you tell me their names, so I have a place to find them if I ever want to look into them?

---

As for traffic, the summary is appreciated, as it was a bit too long (also due to the 12-minute videos) for me to actually commit to reading it. And yes, as someone who often walks in a busy city, that's a lesson I've learned by now... not that it keeps me from trying to speed, both because I like a more difficult walk, and because I've got more options for not having to stand still than drivers and even cyclists (and I barely have that happen at all except for crossings).

_ And that cars which race ahead towards the next red light don't actually gain anything in the long run._

I've also seen that happen with road work, where several lanes need to merge; I don't get the impression that the drivers who race to the end and try to merge there are any faster, and they certainly make everyone else slower.

And that's a very good illustration of this phenomenon!

[igenlode]

I always wrap any emoji that itself ends in a bracket into the closing bracket (since I use a lot of brackets, it happens quite a lot :-p) It looks even odder to me otherwise -- although by reading YouTube comments in Cyrillic I've discovered that Russian emoji conventions do simply consist of multiple levels of nested brackets, e.g. a random) following a word for a smiley and increasing numbers of))) to indicate consecutive levels of amusement! It is very strange to read until you work out what is going on, but I've more or less got my eye in now....

The turbulent theatre director was Yuri Liubimov (or Lyubimov):
https://thetheatretimes.com/century-yuri-lyubimov/
His disciple and commemorator is Veniamin Smekhov, who has been all over my Dreamwidth pages in one guise or another for the last year (at the moment I'm watching a Jack London adaptation in which he played the lead in 1974, and listening to a podcast about a show for which he wrote the lyrics in 1981).
https://sllc.umd.edu/news/2015-2016-maya-brin-resident-veniamin-smekhov
And the film involved was the Soviet-era adaptation of "The Three Musketeers": https://igenlode.dreamwidth.org/373392.html

As for traffic, the summary is appreciated, as it was a bit too long (also due to the 12-minute videos) for me to actually commit to reading it.
Odd, there aren't any videos in the version in my browser. There are a lot of graphs, though!
(Oh, there are a couple of links to MP4 files at the end; I just assumed those did in fact illustrate the conclusions they were said to illustrate.)

The basic summary can be quoted as "Because a significant proportion of the journey time is spent stationary in traffic, and because if every vehicle drives faster or slower, each vehicle still arrives in the same position in the queue at the next set of traffic lights, so effect of speed limit on journey time is minimal. The difference between everyone driving to a 30mph maximum speed and everyone driving to a 15mph maximum speed is about 1mph in difference in average speed."

In London, there generally *is* only one lane, so no scope to 'merge'; normal road width tends to be about three cars wide, which gives you one and a half cars per lane, or a car lane with room for a bicycle to pass. Which is all very well until somebody comes along with one of those fat cars that block the entire lane....

At any rate there isn't a lot of scope for overtaking as a rule. If everyone pulls over there is room for an ambulance down the middle, which is how that works -- although increasingly emergency calls are done by paramedics on motorbikes with special panniers (or even pedal cycles with panniers), since they can arrive much faster through traffic.

[pangolin20]

I think I've seen using more brackets for a smiley in English, too (though it seems to be used more for an ironic smile? I don't have any concrete examples, so I can't say), so this usage won't surprise me now, I think. Having looked up your examples, I can now also see why you'd find the closing bracket weird; it might well cause confusion with a double bracket for a double smile for me, at least. And it's nice to meet someone else who's as fond of using parentheses as I am!

Thanks for the information on these people and for the links!

Odd, there aren't any videos in the version in my browser. There are a lot of graphs, though!

(Oh, there are a couple of links to MP4 files at the end; I just assumed those did in fact illustrate the conclusions they were said to illustrate.)

Ah, that was what I referred to; because these links are put in a paragraph with other text, I considered it as directly part of the article (and they do support the conclusions, as far as I can tell).

That summary fully lines up with what I've seen for myself, so it's nice to have that confirmed by an actual study!

I've never been to a city as large as London, so it's interesting to know what it's like there (which is mostly "very busy", it seems! Ambulances usually get through where I've been). As for merging, I mostly based that off what I've seen on motorways, though there are some two-lane roads in cities I know, like Amsterdam, so I suppose it could be relevant there, too. And it seems I've learned a new word with "pannier", so that's nice to have.

[igenlode]

London has been very busy by all accounts since the Middle Ages - the nature of the vehicles in the traffic jams just changes :-)
But multi-lane roads tend only to exist where houses have been knocked down in order to create them - or else you have a compromise with two lanes in one direction and one in the other, or an extra lane when approaching a junction.
This is a fairly representative view of Central London streets and traffic speed (note supplementary bike lane at the start to allow the cyclists to pass :-D) These would originally have been considered extremely spacious streets when constructed, and if you see them without cars on a Sunday morning you can see that they still are.
The main roads where I live are narrower...

[pangolin20]

Good to know! Looking at some pictures of those streets, they're indeed rather wide for a city centre... I think I only just realised that, even though London is a rather large city, its centre is still the same thing as the city centres I'm familiar with, which makes the congestion much more intuitive to me! A contributing factor is probably that old city centres in the Netherlands are considerably smaller than London's, and that they don't generally have the equivalent of A-routes running through them.

The main roads where I live are narrower...

They probably are where I live, too (though that's partly because most of these roads don't have anything next to them), which further helps to put this into perspective!

[igenlode]

London is weird because it is basically a collection of villages that got merged/swallowed up, each of which has its own original centre; the very middle consists of what were originally two neighbouring cities separated by open countryside, the City of London (St Paul's Cathedral) and the City of Westminster (Westminster Cathedral) -- the English definition of 'city' being 'a town with a cathedral in it'!
So it has *lots* of 'old city centres' on a relatively small scale, rather than one big central area surrounded by suburban housing: places like Chelsea or Hammersmith were originally entirely separate settlements upstream.

But then there was bombing, and clearances to build new roads (most notoriously the Great West Road in Hammersmith, which is a 1960s American-style multi-lane flyover that blights its surroundings: https://metro.co.uk/2025/10/09/unpleasant-hammersmith-flyover-torn-replaced-a-tunnel-24380426/ )
There are still some of the old medieval streets left near St Paul's, but they are just pedestrian alleys, often covered over between office blocks, and not visible to cars.
https://theworkingline.com/secret-alleys-london-streets-hidden-passageways/

[pangolin20]

Thanks for all the new information, as I really didn't know that before now, and it's certainly good to learn more! For some specifics:

London consisting of multiple city centres makes more sense of its structure; I knew that the outer suburbs were once separate villages, but I hadn't considered it for the core, so I did find it weirdly large... and now I know what's causing that.

Yeah, I didn't even need to read the article to guess that it dominates its entire environment. Good to hear it might be replaced with a tunnel, though.

Nice that some of those streets have survived to the present day, even if they aren't obvious now; it's still a bit of history in a relatively accessible place.

[igenlode]

'Pannier', of course, is pure French (panier, basket) -- presumably in the days of our Norman French overlords panniers were originally pairs of baskets slung across the back of a beast of burden :-)
Later they became strong bags used for the same purpose, and nowadays the term is used by analogy for the luggage-bags (or hard boxes, in the case of a motorcycle; possibly those are more rigid because the extra weight is less of an issue with an engine to do the propulsion, or because if the machine falls or is laid down on its side they will get crushed) carried on the back of a two-wheeled vehicle.

[pangolin20]

presumably in the days of our Norman French overlords panniers were originally pairs of baskets slung across the back of a beast of burden :-)

And it's still used with much the same meaning, I see! (By the way, I see that "panier" comes from Latin "pannarium", which means "bread basket".)

Hmmm, it seems to be that they're more durable than soft ones, or that's what sites listing pros and cons of hard panniers tell me.

carried on the back of a two-wheeled vehicle.

Would it be idiomatic to call the bags on a bike that? (If so, that means I can equate it to an existing word in my native language.)