And as has been proved MANY times before, adding road capacity works for a short period, sometimes only weeks, until supressed demand is unlocked. Then the congestion would return. And it increases demand on the roads around it, making overall congestion worse.
The other trend that creates the extra congestion is the desire for more space that is behind the move out of cities into market towns and villages.
Every increase in road capacity will be seen as an opportunity to live somewhere nicer, further away from the workplace.
Adding to that is the mass housebuilding programme. On top of the thousands of new dwellings already completed or being built round my town, there is a new Planning application for another 330 homes on Green Belt land which I am certain will get approval. The Planners are proposing more schools, GP surgeries (where are the additional GPs coming from - Mars?) but NO additional road capacity. Only more traffic lights to let these extra vehicles join major roads without long queues.
The A4 Saltford Bypass is never going to happen neither is there going to be a new link road to the M4 anytime soon, because the land that was earmarked for it has been built on, including a Science Park as well as the Lyde Green estate. Despite the vestigial slip roads for the intended junction, that can be seen when you drive along the M4.
Gridlock is inevitable around Bristol and Bath. Already I avoid driving at peak times. Maybe that's behind the plan to phase out ICE vehicles, in order to get millions of low income marginal motorists off the roads altogether. Restrict their personal freedom. Also turning the hard shoulder into all lane running on Smart Motorways. Because the cost of new roads and widening motorways is so high and the timescale is too long (in the UK anyway). I suppose we were lucky that the M25 was built in the 1980s before the effective moratorium on new motorway building.
Back on topic - the war on bad vehicle emissions was won after leaded petrol was phased out and vehicles had to meet tighter emission standards. I remember that car Ad boasting that the air coming out of the exhaust was cleaner than the air going into the engine. Euro 6 means much lower NOX and particulates. CAZs and LEZs deliver only marginal benefits. So they keep moving the goalposts.