Both Zac Goldsmith and Sadiq Khan claim they’ll be London’s greenest-ever mayor if they win May’s election.

Let’s face it, somebody needs to be, and fast. Air pollution has become the capital’s hidden killer, with 9,500 Londoners dying each year from its effects.

Air pollution comes from a number of sources. Some is blown in from overseas, although also we also export our own. But London’s big culprit is diesel engines. Not just in buses, taxis and HGVs, but in cars bought by drivers told by government they would be more environmentally-friendly than petrol engines.

These engines spew out particulates, which can cause lung cancer and worsen respiratory problems. There’s also evidence that unborn babies’ exposure to air pollution could be associated with low birth weight, poor growth, and an increased risk of chronic diseases later in life.

Without determined action from the Westminster Government – perhaps in helping diesel drivers switch to cleaner fuels – there’s always a limit to what a mayor can do. But outgoing mayor Boris Johnson hasn’t exactly attacked the problem with enthusiasm, with timid “solutions” such as literally gluing pollution to roads and an ultra-low emissions zone that doesn’t extend beyond central London.

Most of his potential successors agree that this isn’t enough. But there’s one area that urgently needs looking at if London is to tackle its pollution problem long-term – Johnson’s roadbuilding plans, starting with the Silvertown Tunnel.

Pollution’s often linked with traffic jams. But anywhere with busy traffic, whether moving or not, is going to be a pollution blackspot. A recent study of a road expansion scheme approved by Ken Livingstone – the A206 at Crayford – showed pollution levels up, even if queues abated for a time.

The Silvertown Tunnel is sold as instant congestion relief for the Blackwall Tunnel. If built, it’d run from the Royal Docks to the Greenwich Peninsula, feeding into the already-congested Blackwall Tunnel southern approach.

There’s a laudable aim – to to relieve the terrible queuing at the northbound Blackwall Tunnel. But new roads have an irritating habit of generating new traffic. The Silvertown Tunnel would worsen jams at other bottlenecks away from the tunnel mouth, on both sides of the Thames.

While both Blackwall and Silvertown Tunnels would attract a small toll, the lack of alternative routes isn’t likely to lead to a drop in traffic – indeed, TfL has predicted 20% more traffic using the two tunnels.

TfL’s plans have led to objections from the operators of the Excel centre as well as from Lewisham, Hackney, Southwark and Waltham Forest councils. Even Newham Council – once a supporter of the scheme – has objected to TfL’s most recent set of proposals.

There’s an outright pledge to scrap it in Liberal Democrat Caroline Pidgeon’s manifesto. Green candidate Sian Berry is also an opponent. Labour’s Sadiq Khan, rightly alarmed by air pollution fears, has pledged a review of the scheme.

But former environmental campaigner Zac Goldsmith, now standing for the Conservatives, is vowing to press ahead with the scheme.

The former Ecologist editor even includes a disastrously ill-conceived concession to air quality fears, pledging to increase tolls for the dirtiest vehicles. Yet all this is likely to do is send those dirty vehicles on detours to the Rotherhithe Tunnel, adding to pollution levels through Greenwich, Deptford, Limehouse and Poplar.

Perhaps in Zac’s mind, clean air is only a right if you live in west or central London. Or maybe he’s being terribly advised. Whatever, it’s a shockingly naive proposal.

The Silvertown Tunnel is just the start. Other roadbuilding plans are slated for Thamesmead and Belvedere (two more river crossings), Hammersmith (the “flyunder” to replace the flyover), Croydon (a new flyover), and a scheme to build a tunneled ring road around central London. All of these have the potential to pile extra traffic onto areas that simply can’t cope with the extra demand.

Nobody is denying that congestion is a problem – but building more and more roadspace won’t do a thing to solve it in the long term. Neither central London nor the suburbs have capacity for an infinite number of vehicles, despite what big developers and some borough councils might think.

The mayor can wield stronger tools to deal with congestion and pollution – expanding the puny ultra-low emissions zone, widening the congestion charge, boosting public transport as well as walking and cycling.

Boris Johnson hasn’t been the only one asleep on the job. Too many of London’s borough councils, of all parties, have also encouraged schemes that will encourage extra pollution and congestion.

But whoever moves into City Hall next week has the chance to make a fresh start and show some leadership – particularly as there’s none coming from Westminster. Scrapping the Silvertown Tunnel would be a signal that the new mayor is capable of it.

For information on the campaign against the Silvertown Tunnel visit silvertowntunnel.co.uk