I visited it last year. It’s 2.4km long and at its highest point the Eiffel Tower could fit under the road. Remarkably the construction cost was only €394 million.
For comparison the planned 4.2km Lower Thames Crossing has already cost £1.2bn (€1,400 million) just for the planning phase with nothing built. The French know how to build.
While our construction costs are indeed ridiculous, this number is incorrect. It hard to decipher which £1.2bn figure you are actually talking about - but none of them are for just planning (for reference: the contract for the northern connecting highways and the contract for the actual tunnelling are both for a similar amount of money; the total spend as of 2025 is also around the same amount but it includes initial payments on all contracts etc).
I got the 1.2B from the Beeb: https://www.bbc.co.uk/news/articles/c0mrm84wg4ko.amp
> Plans for the 14.5-mile (23km) route were approved in March after a 16-year process that has already cost £1.2bn.