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Back in the days when Sherlock Holmes was clattering around London’s smoggy streets in horse-drawn hansom cabs, the future almost arrived to replace them.

In 1897 primitive electric taxis were tried out in the city. They didn’t last long: their weak batteries were no match for powerful petrol and diesel engines. Now, more than a century on, electric cabs are getting a second chance — and this time the fight to clean up London’s air quality needs them to work.

You can already spot a new generation of slick electric black taxis purring their way around the city — vehicles that can recharge in an hour, cover around 80 miles without emitting anything worse than the honk of a horn, and turn to a small petrol motor to top up the battery on longer journeys.

They are one of the slickest parts of a huge battle to clean up London’s air, which reaches a new level on April 8, when the Ultra Low Emission Zone — Ulez — comes into force. It will charge the most polluting cars, vans, buses and lorries extra to enter the congestion charge zone in central London.

Curiously, diesel taxis, which account for about 20 per cent of emissions of harmful nitrogen dioxide, won’t be made to pay — just as they don’t have to pay the congestion charge now. Cabbies are being encouraged to switch to the new electric vehicles and given grants to help them do so, but some might ask why some of the biggest polluters in central London are being let off the new charges which everyone else faces.

The plan for the Ulez is “the most radical in the world”, says Alex Williams, director of city planning at Transport for London (TfL). “It’s been in development for five years.” Announced by Boris Johnson when he was Mayor, it has been pushed forward by his successor, Sadiq Khan. “The difference between this Mayor and his predecessor is about pace and urgency,” Williams says. The Ulez was due to arrive in 2020. “Within days of coming in the new Mayor was very clear that he wanted to accelerate that.”

So from next month everyone who drives a diesel car that doesn’t meet what are called Euro 6 standards for diesel, and Euro 4 for petrol, will have to pay £12.50 on top of any congestion charge if they enter central London. That means most diesels sold before late 2015 will be priced off city roads. Most small vans sold before September 2016 will have to pay the same.

For lorries, coaches — and even fire engines and gritters — it will be £100, which might be a challenge for the London Fire Brigade, 60 per cent of whose fleet doesn’t comply, although vehicles actually based in the central zone do.

“Those who can pay will pay but it is going to be a horrendous amount of money,” admits Professor Frank Kelly, at King’s College London, whose research team carries out the scientific work that shows the extent of the capital’s air quality crisis. Last week a high-pollution alert was issued for London, despite the sparkling weather. “It’s easy to say you can’t see pollution but we must deal with it,” says Shirley Rodrigues, City Hall’s deputy mayor for the environment. Modelling shows the central Ulez will reduce levels of nitrogen dioxide by 45 per cent in the first year.

"Any surplus money that is generated will be reinvested into air quality" Shirley Rodrigues

The aim, those behind the plans promise, is to drive old polluting vehicles off the streets. It’s an issue of fairness, argues Rodrigues, since bad air quality most affects the very poor, the very young and the very old. “Any surplus that is generated is reinvested into air quality,” she promises — adding she’s “very confident” the scheme will come in smoothly this April.

But that won’t stop a big shock in October 2021 when the Ulez is set to balloon, taking in all of London inside the north and south circular routes — which is meant to cut nitrogen dioxide pollution by a further 23 per cent.

That means millions of people will be hit. From Clapham to Wood Green and East Ham to Acton, it will span London’s roads, cutting across many borough boundaries — to the anger of some.

“We have just got to get fewer vehicles on our roads,” says Professor Kelly, who praises the Mayor for taking action and says the Environment Secretary, Michael Gove, is leading too. “Before, there was no previous minister who had any real ambition in this area,” he says. “Gove came in and he has made a difference.”

But will those who have to pay agree? Van drivers are having to replace almost-new vehicles and many people who bought diesel cars because they use less fuel and so emit less carbon dioxide, helping the fight against climate change, are being told to pay — even though it was central government which backed diesel in the first place. There’s a £23 million scrappage scheme planned for vans and Transport for London is pouring cash into new buses — all single-deckers are going electric and an electric double-decker is being developed too. Meanwhile, cleaner diesels are running on the most polluted routes.

The backlash has begun. The Conservative mayoral candidate Shaun Bailey says he wants action on air quality. But on the Ulez he’s chasing votes: “People using cars in outer London need them, they use them because they don’t have a choice and we need to get rid of the Ulez in outer London,” he said last month.

We’ve heard it before. Mayor Ken Livingstone brought in the original congestion charge in 2003 — a way of raising cash and boosting public transport — but its extension into west London was a target for his successor Boris Johnson, who scrapped it when he took office in 2010. Johnson then had a change of heart, backing the central London Ulez as concern about air quality grew, encouraged by polling that showed Londoners want action to protect children’s health.

Opposition to the Ulez now is “incredibly depressing” says Matthew Pencharez, who was deputy mayor for the environment when Johnson was in office. But he makes the point too that focusing only on air quality can divert action from other issues, such as climate change or funding road safety schemes to help pedestrians and cyclists. And people in outer London do depend more on cars than elsewhere: telling people to walk or cycle isn’t always an answer.

There are other sticky issues too. “Heathrow is a can of worms,” says Williams at TfL. The Mayor is facing a court challenge this month about the impact expansion might have on air quality. Right now, though, those in charge are focusing on getting the message across about the first stage of the scheme and making sure it works. Already several hundred thousand letters have been sent to drivers spotted in the zone, warning them that their vehicles soon won’t comply.

“We’ll see a ramping up in coming weeks,” says Paul Cowperthwaite, TfL’s charging boss. The plan is for eight out of 10 drivers to comply with the new rules from the start — enforced with existing tracking systems used for the congestion charge. But people will need to be told they are entering the zone. “Road signs are being installed to be uncovered on day one,” he says.

What then? Will fewer drivers and cleaner buses in central London lead to other shifts? Westminster council has just published plans to cut traffic on Oxford Street, one of the most polluted roads in the city, which fall short of ones backed by TfL. Private cars are already banned, so the Ulez shouldn’t help much. “It is so backward,” says Professor Kelly, who monitors its air quality. “My ultimate vision is we have a pedestrianised Oxford Street — in the whole centre of the city there is no need for vehicles during the day.”

Winning that battle will be hard. “There’s a real challenge for the Mayor to be continually explaining the idea behind this,” says Kelly. “You see him in schools but he needs to widen his horizon.”

The test will be whether the new charge comes in smoothly, or whether people and businesses think they are being singled out. Any wobbles next month as part one of the Ulez comes in and the big expansion two years later will be in opponents’ sights.