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]

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.