HOP on the number 15 bus at Cannon Street, and the last one-and-a-half miles of its route to Charing Cross can take more than 25 minutes, even outside rush hour. The streets are littered with notices advertising current and future roadworks. Traffic in the capital is abysmal, says the bus driver, who points out that unlike taxis he cannot swerve down side-streets to avoid it. Small wonder London’s buses are losing passengers.

Buses are the busiest mode of transport in the capital. Two-thirds more journeys are taken on them than on the Underground. Until 2014 passenger numbers had been growing steadily for about a decade. But in the past three years they have fallen by about 6%. The decline comes even as the capital’s population continues to grow and employment rises. Whereas more people travel on the Tube and the Docklands Light Railway, they are stepping off the buses.

The congestion that so enrages bus drivers is partly to blame. Growing fleets of delivery vans are clogging up London’s roads, which are dug up more often than those of some other capitals (see map). Between 2012 and 2015 disruption from planned roadworks increased by more than 360%. As a result, buses are travelling more slowly. Speeds have fallen from an average of 9.7mph (15.6kph) in 2013-14 to 9.3mph in 2016-17. Use has fallen fastest on those routes with the biggest drops in speed. On those where speeds are down by more than 8%, use is down by 16%. Some people are switching to the Tube.

Smartphone apps that give real-time updates may also play a part, reckons Leon Daniels of Transport for London, the transport authority. Apps like Citymapper make the bus network easier to navigate, but knowing a bus is ten minutes away may mean a person walks.

Buses are victims of the well-intentioned war on cars, suggests Tony Travers of the London School of Economics. The roadworks clogging the capital’s arteries are the result of efforts to get people cycling and walking instead of driving. London has been building bike lanes, wider pavements and four-way crossings to improve life for pedestrians and cyclists. But such changes come at the expense of roads—and buses.

Does the decline matter? Transport is not an end in itself, argues Mr Daniels: “Our job is to cater to those who want to travel. We’re not selling goods.” But inefficiency is expensive. Congestion means that more buses are needed, since each bus makes fewer journeys. Each costs around £350,000 ($460,000) a year to run. Clear the roads, and fewer passengers might give buses a pass.