Roads are hideously expensive - but smart motorways aren't just wifi enabled signs: there's traffic/weather detection and then the control systems behind adaptive traffic lighting/speed control - and those systems have to be reliable and maintained.

I haven't seen recent statistics/costs but remember that the cost of the converting the M42 in 2006 to a smart motorway was £9M per mile, creating a new lane would have been £16M per mile.

https://en.wikipedia.org/wiki/Smart_motorway has a list of miles and costs.

In the days when I can buy a weather station for $16 or get the weather in my phone for free, I don't think this should add much to the cost.

Likewise I can get bulbs that turn themselves on and off based on weather, light, the stock market or the phase of the moon, all for $3 from China. There is no way someone should be allowed to charge the UK government millions for the same.

I doubt the $3 weather sensor has been tested / rated / secured in the same way that sensors for a motorway would be.

My guess is there are huge profits for the companies that build them.

Some of the motorways in the UK have been under construction for what feels like decades. Surely the same job could be done with machine vision these days!

And they’ve been blocking roads with highly disruptive roadworks for literal years to “convert” them