what unusual (but not cruel) punishment do you think would work to deter certain crimes?

No we don't. The UK road network is ridiculously congested, partly because there isn't the space to build additional capacity in most towns and cities. Lots of streets, and the buildings on them, date back decades or hundreds of years, are narrow and are not designed for modern road use. This is why bypasses are a thing. Look at the vast majority of places that bus or cycle lanes have been added to the existing road network, either by replacing an entire running lane, or by carving out a small strip at the edge for a cycle lane. Half the time, bus shelters get built directly adjacent to the road, instead of having a layby for the bus to pull into - because there's no room. You talk about widening roads, half of those schemes never get off the ground, because they're too difficult to achieve in a built up area. The North and South Circular roads in London are perfect examples. And how many cars would each layby be able to hold? If they reach capacity of "a couple of cars" then you've blocked the road for other traffic. Even if you had the space (which you don't), the cost of converting thousands upon thousands of junctions and installing cameras everywhere would be prohibitive. Look at how much TFL is spending on ULEZ cameras, and then extrapolate that across the entire country, just for the cameras. Junctions would have to be redesigned as left-hand turn lanes would cross over traffic trying to rejoin the carriageway from these laybys. You would have major problems with traffic joining from junctions not controlled by traffic lights, leading to gaping holes in the camera network, rendering it ineffective. Even if you can solve these problems, what you're creating is a gigantic ANPR surveillance network. Honestly, this idea has not been thought through, at all.