The marvellous thing about reclaiming London's streets for human beings is that a wide range of human beings approve. Public squares, high streets and residential roads where it is easy to walk, safe to cycle and just pleasant to exist are desired by environmentalists, pedestrians, businesses, shoppers, public-transport users, planners, bike-riders and property developers alike. The consensus is neither perfect nor complete, but its embrace is broad and deep. For much of this century the capital has travelled strongly in its direction. It could and should go further. Lately, though, its compass has strayed.

Making streets more "liveable" depends vitally on calming and restricting their use by private motor vehicles. Monday's announcement by Transport for London that it's consulting on raising the congestion charge was a reminder of the ways Boris Johnson has, to strain a metaphor, shifted the city into reverse. A second increase in the charge under the Conservative mayor would be welcome but should be judged in the context of his halving of the charging zone at the end of 2010, a move which not only invited more cars, vans and HGVs into west central London but has cost TfL, by its own estimate, around £55m a year.

This was not the only early Johnson measure to end or undo the reforming programme of his predecessor Ken Livingstone. Plans to pedestrianize Parliament Square and introduce a tram service linking north and south London by way of Waterloo Bridge were dumped soon after he took power in 2008, as was Livingstone's "modal hierarchy" which prioritised pedestrians, cyclists and bus-users. Since then, traffic lights have been re-phased to help drivers get around faster but there are no plans to increase bus service capacity. Johnson, although an impressively visible cyclist, has suffered mixed fortunes implementing his various shades of "cycling vision." The latest, though well resourced, will have the look of a face-saving indulgence detached from any broader perspective until proven otherwise.

The mayor's transport strategy, always a bit of a dog's dinner, seeks a large increase in journeys on foot, but a planned Year of Walking (2011) never happened and the London Assembly's transport committee heard on Thursday that "green man" time has been shortened at many busy crossings with no thorough analysis of its effect on collision rates.

In his defence, Johnson can point to TfL's road safety action plan published last year. But much attention is centring on recent casualty stats. Adam Bienkov and the Green Party's Jenny Jones have calculated that the rate per journey at which cyclists have been killed or seriously injured has gone up in recent years when the opposite might be expected to apply under a mayor whose use of pedal power is so central to his jovial persona.

Johnson has denied that the same trend holds for pedestrians, and, in truth, this is hard to calculate. TfL points out that the numbers killed and badly hurt in 2012 (the most recent year for which data are complete) are down compared with the 2005-09 average but also acknowledges that it was a very bad year, showing a sharp rise compared with 2011.

It would be wrong to oversimplify the bigger street management picture across London as a whole. Led by Islington, there's been a strong move towards the boroughs, which are in charge of the great majority of London roads, introducing 20 mph speed limits. The mayor's transport deputy Isabel Dedring, who led his roads task force, has said these could apply to almost all the capital's high streets and residential roads by 2020. That task force has also managed to keep alive the case for further congestion charging.

In Hackney, there is great pride in the priority given to buses along with measures to help both cyclists and pedestrians without putting the former before the latter, as some new cycling infrastructure is claimed to do. In Kensington and Chelsea, the redesign of Exhibition Road, whilst a disappointment to some, does show the potential of the "shared space" principle.

It will not do to glibly blame every negative trend in this vital area of transport policy on the mayor – it may, for example, suit his Tory instincts to shift the conversation towards individual responsibility, but that doesn't invalidate the case for it. Yet his approach looks piecemeal and half-hearted at best when compared with imaginative recent programmes in New York or the Vision Zero pledge on traffic fatalities made by that city's new mayor Bill de Blasio. Johnson has allowed the momentum towards a more "liveable London" created by his predecessor to become diluted and diffused.

In 2004 the Danish architect Jan Gehl produced a report, co-commissioned by TfL, containing imaginative recommendations for gradually transforming London into a place where a better balance between road traffic, cyclists and, in particular, pedestrians worked for the good of the city as a whole. Welcoming the report, TfL's head the time, Bob Kiley, wrote of the key role Gehl had played in "the transformation of Copenhagen city centre into a lively and prosperous place where people can move and meet in comfort and in safety." Gehl's ideas, he went on, "have been exported around the world with similar extraordinary results: now it's London's turn."

Ten years on, Gehl is to return to London to attend a screening at the Hackney Empire theatre on 23 January of a film about his work – entitled The Human Scale – and participate in a panel discussion afterwards. The panel will – full disclosure – be chaired by me and also include Patricia Brown, who has helped bring about some of the best improvement to London's places and spaces and was another key backer of the 2004 Gehl report. There may be some tickets left but you'll have to move fast.

The event will, I hope, help place the debate about London's streets right back at the heart of political debate in this year of borough elections and with the next mayoral poll in 2016 already on the horizon. It's a vital debate as well as sometimes a difficult one. While there are many common interests in taking the Gehl approach, they aren't always neatly aligned. The right mix of measures is the key, but the politics can be complex. And there remain powerful opponents of any attempt to curb the privileges of the private motor car, as famously demonstrated by the Evening Standard's extraordinary response to the idea of weekend and evening parking charges being levied in Westminster. It's time to renew the argument for change.