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Very important campaign against another motorway link in London:
http://www.lfgss.com/thread122437.html
Don't be fooled by it being in a tunnel, the drip-drip of road schemes like this extremely damaging. No more urban motorways.
There's another tunnel scheme being promoted here - put the A4 underground:
https://www.lbhf.gov.uk/Directory/News/Hammersmith_flyunder_options_to_be_revealed.asp#4The A4 and Hammersmith gyratory blight the surrounding area so there is a certain attraction to sticking them underground and out of sight... but this would mean spending billions on car journeys into central London just so they can crawl along at a few mph.
LBH&F don't give a shit about cycling but I'll bet the developers are crawling all over them about this scheme given the potential to release land for development.
I can't see it happening - the Chiswick NIMBYs will be out in force against the big fuck-off Crossrail-like construction that would be needed in W4.
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That's the theory. Putting the A4 underground would release a strip of land that could be 2-3 miles long and presumably developers would be queuing up to build on it.
There doesn't appear to be a proper study on the economics and feasibility yet, however. Surface roads would still be needed for access to all those new developments and anything involving tunnel boring machines will mean big construction sites at the ends.