Newly built wider roads would be a tiny proportion of the total as compared to the previous "unwidenable" roads, so making new roads wider would have little eff

Newly built wider roads would be a tiny proportion of the total as compared to the previous "unwidenable" roads, so making new roads wider would have little effect. It would be much more efficient to more strongly regulate/restrict/tax the width of cars so that they are the appropriate size for the vast majority of already existing roads in the UK.