You often see the comment "all roads are for cars". This is the wrong way of looking at it - roads are designed for a certain purpose. Even in a London suburb from the early 20th century you can work out a road's purpose. Also roads have a handy classification to help work it out

'The system of roads classification is intended to direct motorists towards the most suitable routes for reaching their destination.' 'A' roads are strategic routes or main roads 'intended to provide large-scale transport links within or between areas.'

Unclassified roads are 'local roads intended for local traffic.' While these are official classifications of course we live in a borough mostly designed before mass car ownership and mostly before the classification started in the 1920s.

However had planners realised there would be a period of mass car ownership - as they realised post-war - they would have designed residential estates as cul-de-sacs. So LTNs are just a series cul-de-sacs with surrounding A roads.

What is the problem with using unclassified roads as main roads? It all comes down to the road design and A roads should be designed to deal with the interactions between vulnerable road users and motor vehicles. So traffic controls, crossings, pedestrian refuges, cycleways etc.

And the result is that they are safer for all. But what happens when we open up unclassified roads? Some research by @RachelAldred shows that KSIs go up by 17% and minor injuries go up by 66%. A great thread here. https://twitter.com/lambethlivingst/status/1318184666088824834?s=21

Importantly motor vehicles 'drive out' active transport on unclassified roads eg less walking and cycling. Not only is the design of unclassified roads wrong but the data shows that speeds are too high on these roads too. There is even data from the Fox Lane scheme to show this.

For pedestrians and cyclists it could just be a perception that is more dangerous, as well as the reality, but it is enough to deter active travel. A joined up series of LTNs with crossing points would lead to an increase in active travel. A trial is the best way to show this.

This is a 20th century problem which, post-war, we've known of a solution but only now, in the 21st century, are we tackling. So now we need to retrofit our cities/towns for (hopefully dwindling) mass car ownership. Roads should be used as they were originally intended.

P.S. Indeed as well, LTNs could lead to decreased car ownership. https://findingspress.org/article/17128-low-traffic-neighbourhoods-car-use-and-active-travel-evidence-from-the-people-and-places-survey-of-outer-london-active-travel-interventions

P.P.S. But just to come full circle you will be happy to discover that there are some "roads [that] are for cars". Motorways are for motor vehicles - because that is their purpose (and we are not asking for low traffic neighbourhoods on them).