Roads! Everyone loves roads, right? With their cars and their tarmac and their air pollution and that. Vroom vrooooooooooooom.

One man who has definitely decided we should be building more roads is London mayor Boris Johnson. This week he’s in Boston and New York, because reasons, and has decided now is the perfect time to announce a clever plan to start burying bits of London’s trunk roads.

This is not as a crazy and idea as it might initially sound. They did it in Boston, byreplacing a six-lane elevated highway with an eight-line underground one. They’re doing it in Hamburg, where the autobahn is being replaced with a park. They did it in that episode of Pigeon Street, too.

Sticking big roads in the ground has a number of advantages. It can reconnect neighbourhoods currently severed by main roads. It can make cities more attractive, if only because almost anything is more attractive than a great big bloody motorway.

And it can free up land for other uses – housing, office space, parkland, and so on. That, we suspect, is one of the reasons London’s government is currently leaping on the idea. Not only does this mean more space to cope with London’s population growth, but, with land values so high, the development opportunities will make any project a lot easier to pay for.

This morning City Hall said in a press release that it had examined “more than 70 locations… where the introduction of tunnels, fly-unders and decking could deliver benefits”. That number looks a bit optimistic, though, and only five have been identified as “suitable for further feasibility work”; and several of these don’t sound terribly ambitious.

Nonetheless, here they are:

1) “Decking or a mini-tunnel” over the North Circular in New Southgate;

2) Turning the Hammersmith flyover on the A4 into the Hammersmith tunnel;

3) A “small fly-under” on the A316 at Chalker’s Corner, where it meets the South circular near Mortlake;

4) “Decking of the A3 in Tolworth”;

5) “A mini tunnel of the A13 in Barking Riverside”, to connect one of London’s biggest potential housing developments with something that resembles civilisation.

Here’s a map, on which we’ve essentially highlighted the proposed developments in red crayon. (In an attempt to make them visible, our versions are almost certainly bigger than the real thing would be.)

To give a sense of the full scope of Boris’ petrolhead ambitions, we’ve also included the three proposed East London river crossings: the Silvertown Tunnel, and Gallion’s Reach and Belvedere bridges.

The really big one, though, is one Johnson has been talking up for a year: the proposal to create a whole new ringroad underground at a cost of £30bn.

What that would look like is not exactly clear. This is the version that was floating around when TfL first broached the idea last spring…

Image: TfL.

…but that doesn’t seem to fit with TfL’s response to a Freedom of Information request made by the blogger known as Boris Watch. That suggested the tunnel would be 70km long – roughly twice the length of the network shown in the map above. It also lists 10 junctions, including one labelled A10/A503, and another A23/A205, which strongly suggests that the loop would go as far north as Seven Sisters and as far south as Streatham Hill.

If that’s accurate, then it seems likely the final tunnel would look a lot more like this. (We’ve labelled the junctions included in the modelling.)

The bottom line is that we don’t actually know; neither, we suspect, does Transport for London.

But what is clear is that, for the first time in a generation, expanding London’s road network is seriously back on the agenda – for better or worse.