I am posting this on behalf of Kerry Wood, who describes himself as a retired engineer with an interest in transport and sustainability.
The quality of peak-hour travel by car tends to equal that of public transport.
This simple observation is a formal statement of the ‘Downs-Thompson effect’, a useful way to think about traffic and ‘car dependence’.
London’s M4 motorway opened on a Saturday in 1965 and came to a standstill on Monday morning. It did not relieve congestion on the parallel Cromwell Road. The same thing had happened when the Cromwell Road Extension opened 30 years earlier.
Downs-Thompson effects don’t always come as quickly as this, but come they will. They usually wipe out half or all the expected benefits within three to five years. Other names for the process are induced traffic or triple convergence: existing trips converging from other times of day, other routes and other modes.
- Another time of day may be as simple as leaving home a few minutes later, or not waiting so long for homeward traffic to ease off. The result is a shorter and more intense traffic peak.
- Another route may be a switch from back-streets to the new highway, or to another destination. It may be a more distant workplace or shopping centre, or a new home more remote from the same destination.
- For a new mode, the most common change is to using the car, or using it more often.
While there is real traffic growth, most growth is existing trips filling up the new road space. Triple convergence is a drag on the economy, delaying everybody but supporting only car and petrol sales.
Some drivers will always use their car but others actively explore their options. As triple convergence progressively slows traffic they may try the bus again, but find it worse than ever.
- Delays in re-entering a fast-moving traffic stream on leaving a stop.
- Consistently missing traffic signals set to give motor traffic a ‘green wave’.
- A more indirect route to pick up passengers, such as bypassing a flyover.
- Degraded access because a busy road is hard to cross and stops become inaccessible.
If there is a parallel rail service the ‘explorers’ will also try the train, but may find it is also worse. Operator responses to lost traffic may include higher fares or less frequent services. Older trains may have been neglected and become unreliable. By the time road traffic stabilizes—at the same overall speed as travel by train—the train service may itself be degraded. Everybody is worse off, unless subsidies have maintained standards.
The pressure eventually builds up and something is done. Suppose that this time the railway is chosen: greater capacity on faster, more comfortable and more reliable trains. Now some explorers find the train suits them best, and some non-explorer friends may also be convinced. As more drivers take the train, road traffic speeds up until the quality of peak hour travel by car again equals that of public transport.
In reality there is no simple either-or here. Traffic surveys in London have shown that about 15% of cars in a morning peak are missing the next day, but the total number is much less variable. Train users often take the car sometimes, just as drivers sometimes take the train. A good central area cycle-hire scheme may remove some of the reasons for taking the car. Other options include car-sharing, cycling, school buses and tele-working.
Trucks and diehard car-users do not need more road capacity: there is already space for them. The question is how to keep it clear for those who need it.
The standard solution is queuing: the most inefficient method there is. Who was it described roads as the last refuge of Stalinism? Road users like to believe they have paid for congestion-free travel, but they are not stuck in traffic: they are traffic. Traffic behaves the way it does because users can game the system; what is the economic value of an extra two minutes in bed?
Nothing can be done while road users make trips having external costs greater than the internal benefits. Fixing it needs not RoNS but road pricing: far cheaper, far more effective but politically unpopular. In Britain, the Royal Automobile Club now supports road pricing.
What can be done to introduce it here?