Famously the son of a bus driver, new Mayor Sadiq Khan risks overseeing a fatal decline in passenger numbers as London’s congested roads take their toll on reliability.

The Mayor, who will be hosting an international bus summit later this year, has been warned that he may have to cut frequency to free up road space, improve speed and reduce pollution.

The London bus fleet regularly bucks the national decline trends but passenger numbers have stalled as journey times increase – by more than 50% in 50 years in urban areas – meaning its quicker to walk in some cases.

Former Transport Commissioner Sir Peter Hendy, who left TfL to head up National Rail, waded into the debate suggesting the new Mayor had left himself little room for manoeuvre with his commitment to freeze fares.

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He said: “Increasing costs and decreasing revenue inevitably lead to less service in London. Fewer but faster and reliable buses will both solve an acute financial problem for Sadiq Khan – the combination of his fares freeze and the complete removal of subsidy from TfL by 2018 – and restart bus passenger growth.”

The Mayor’s office has said savings will come from financial efficiencies within TfL.

Sir Peter suggested that the future was for buses to have the same sort of “protected road space” as cyclists, although his critics argue cycle superhighways, which removed up to 25% of road space on key routes central London, have only compounded the problem for buses.

In a new study, The Impact Of Congestion On Bus Passengers , transport expert Prof David Begg concludes that congestion could entirely destroy the bus sector.

Mayor of London Sadiq Khan speaks to supporters as he arrives for his first day at work at City Hall

He said: “This is a dire and sensational prediction, but the evidence uncovered in this research leads to no other conclusion.

“On historical, current and future trends it’s a question of when, not if. There is a distinct trend across our most congested urban conurbations in the UK of bus journey times rising by – on average – almost 1% per annum.

“If we had protected bus passengers from the growth in congestion there would arguably be between 48% and 70% more fare paying bus passenger journeys today.

“If the trend is allowed to continue, then our urban buses will no longer represent a viable mode of transport for the majority of its customers and will be populated largely by people with mobility difficulties. Already in London some buses on some routes run at close to walking speed.”

Prof Begg says that measures to protect London buses, including congestion charging, cashless buses, red routes and better passenger information have seemingly run out of steam and that a third of bus routes are seeing a decline in journey times more than five times the national average in the past year alone.

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“All the laudable ingredients cannot offset the rapid deterioration in bus journey times.

“The new Mayor has committed to a fares freeze which raises the question of who is going to pay for bus services in London if it’s not coming from the taxpayer as passengers will not make up the difference in higher fares.

“The solution is to operate buses more efficiently by improving their speed. If London is to eliminate the £461million per annum subsidy to its bus network then bus speeds would have to improve by 24%.”

A spokesperson for TfL said: “No cuts to the bus fleets have been considered or discussed with the unions.”

One of the Mayor's first moves in office was to introduce the one-hour hopper fare , meaning people can take more journeys on different buses for the same price.

London’s new Transport Commissioner Mike Brown said: “For many people catching more than one bus is the only way they can get from A to B. This fare will enable us to better meet the needs of those Londoners who live or work in areas which aren’t as well served by Tube or rail services.”

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