Briefing: Do we really need expensive bus lanes?

The City Deal proposes bus lanes on long sections of several major routes into Cambridge, but these will be both expensive and controversial, and early delivery will be difficult. It is important that buses don’t get delayed by slow queues of private car traffic, but is there a more cost-effective way of delivering benefits? I hope I can show there is, and that it works elsewhere.

Both Milton Road and Histon Road provide obvious obstacles to otherwise effective bus routes down the Busway, from the Busway, nearby villages and Park & Ride, but bus lanes through much of their length would be extremely costly and provide little extra capacity on many sections.

It is not the capacity of the roads that is the problem but the capacity of the junctions at the ends. Reduce the flow on those sections of road and there would be no queues to and from the junctions and a huge reduction in bus delays.

So why are bus lane schemes so expensive? Buses are heavy. The damaging power to road structure of a single loaded bus is more than that of around twenty million bicycles! (The damage is proportional to the fourth power of the wheel weight: say 60kg and 4 tonnes.) This means that unlike for a cycle lane or footway, miles of services such as water, gas, electricity, telephone or cable would need to be moved or specially reinforced before any other work started. ‘Statutory Undertakers’ work can take months of disruption and cost many millions.

If any compulsory purchase of private land is needed that can add years to the timescale, and should trees and grass verges need to be removed outside private properties local opposition can be forceful.

So how do we achieve the objective at a lower cost?

Zurich does it city-wide, but SCOOT, an Urban Traffic Control system already in use in parts of Cambridge, can do it locally. It is called ‘gating’ or queue relocation:

‘Traditional bus priority measures have failed to provide continuous priority along congested links and through oversaturated junctions. Queue relocation techniques have shown significant benefits in dealing with these issues and have been introduced at a number of isolated sites.’

We do have similar systems on some busy motorways and it has been long in use on highways in the United States where it is called ‘ramp metering’. After these confusing terms you need a simple explanation:

‘We count them out and we count them in’

On motorways it was realised that when congestion occurs capacity drops further from that achieved in ‘free flow’, so regulating flow can, perhaps paradoxically, increase throughput (a lot of traffic theory and practice in traffic management is paradoxical). On motorways this is achieved by placing traffic lights on slip roads, and restricting access to the motorway if a critical level of flow is achieved. It works!

On the narrow sections of Histon Road and Milton Road when the traffic is free-flowing, as it is for much of the day and all of the night, no significant delays will occur to buses so no bus lanes are needed.

Nearer to the A14 Northern Bypass there are wider sections of road with grass verges and few adjacent private properties. By creating sections of bus lanes here with perhaps two lanes of other traffic that can be delayed, we can ‘regulate’ (gate?) that other traffic into the narrow sections in such a way that no queues form and buses have a smooth and uninterrupted journey to the city centre, or at least as far as Arbury Road and Victoria Road. Between the A14 and Brownlow Road on Histon Road and The Golden Hind on Milton Road there is space for some local widening (expensive highway engineering including removal of trees and grass verges, but not, in general, outside private houses!). This could allow temporary stacking of up to 400 cars while allowing free passage of buses. It might not seem many, but should result in a very significant improvement in bus reliability.

Of course there are other issues, and a major one, although apparently remote, is the M11 or A14. Tailbacks down slip roads onto Highways Agency fast roads can result in serious crashes, and at busy times the use of ‘queue detectors’ on such slip roads may result in traffic leaving such roads having priority over other traffic. I think this is the reason for such serious morning peak queues down Madingley Hill. Once queueing on Madingley Road reaches junction 13 of the M11, it seriously reduces the capacity down Madingley Hill. The Highways Agency could veto any traffic management system if it was feared that slip road queues would increase.

Less of a problem to solve is ‘rat-running’. Drivers will take routes that are longer in distance but shorter in time, often on unsuitable residential streets, to avoid queues. It may even take them longer, but they feel less frustrated when moving than when stationary. ‘Filtered permeability’, where access is available only on foot or bike (or even bus) would be required at a few locations, and a benefit might be gained by re-location of some existing restrictions. It would take a surprisingly small amount of private car traffic to be ‘gated’ for a surprisingly small amount of time to give big advantages to public transport.

Any advantage given to buses should result in win-win-win strongly positive feedback loops:

  • more reliable buses with less delay, means more services can be run with the same number of buses hence increasing frequency with little added cost
  • more reliable buses with less delay, and greater frequency, makes the service more attractive to drivers of private cars
  • more bus passengers with fewer cars means more reliable buses with less delay and perhaps lower fares.

The current situation with more private cars resulting in more delays and less reliability for public transport reduces the attractiveness of public transport, resulting in strongly negative feedback loops. Work here cannot solve all the problems of congestion, but increasing peak hour bus reliability from, say, 80% to 95% should bring us closer to a tipping point where public transport or bike is the obvious mode in and around Cambridge.

Jim Chisholm

More info.

In the early ’80s Jim Chisholm helped for a while in the team that developed SCOOT. There is a short on-line video of the ‘Zurich Traffic System’ at: http://vimeo.com/111711267 with a longer version at: https://vimeo.com/108884155

There is a PDF of an article regarding the principles of ‘gating’ using the ‘SCOOT’ Urban Traffic Control System at: http://tinyurl.com/llybb4h (Advice Leaflet 2: Congestion Management in SCOOT)