part 2 of Jim Chisholm’s supercycleway article
This article shows how better cycleways may be justified using conventional cost-benefit analysis. It first defines some simple principles and then uses more complex arguments to expand these ideas. The article results from ideas about a ‘No Cycling’ day in Cambridge, and from work done by Dr Martin Mogridge on the effects of public transport investment. It makes no attempt to include benefits due to better health or pollution reduction. Newsletter 17 described a low cost network of such cycleways in Cambridge, which could form the basis of a supercycleway.
How congestion happens
When vehicles travel along a link or through a junction, delays may occur. In general these delays will be small until the ‘capacity’ of the link or junction is reached, then they will rise very rapidly and queues will result (see graph below). Capacity is reduced in dark or wet conditions, so queues are often longer on wet days and in the winter.
When there are queues, the actions of individuals can increase the delays to ALL following vehicles. For example: if at 08:00 a single driver stalls at a junction and has difficulty restarting, resulting in a delay to him of 30 seconds, this will also delay all subsequent drivers an additional 30 seconds, until the queue ceases to exist. If the queue exists until 09:00, and 1 200 people (include often forgotten bus passengers!) pass in that time, the total extra delay caused to others would be TEN HOURS. Similar effects would occur if say 6 to 10 new car journeys were made passing at 08:00.
A measure of the ‘elbow’ effect of the flow delay curve is that vehicle traffic in the ‘peak’ hour over half term declines by only some 10% (see table) and yet that is sufficient to remove almost all queues in Cambridge over these weeks.
|half-term week||term-time week||% change|
- if the peak traffic in Cambridge could be reduced by a mere 10%, delays would be reduced by something approaching 90%
Marginal cost and social cost
When individuals make a journey choice they choose a route and mode of transport (walk, bike, bus, etc.) which they perceive as having the lowest cost. Early traffic models assumed that these costs were just the sum of ‘cost of time’ and ‘perceived cost of travel’ (petrol, fares, etc.). Now it is realised that many other factors need to be included. Safety, reliability of arrival time, waiting and convenience all have value to individuals, and these will be included in the traveller’s cost. Especially for car trips, this normally means the driver’s marginal cost, and no account is taken of the fixed costs. You can do a large number of bus, train and taxi trips and even hire a car a good few times before the cost equals the tax, insurance, depreciation, and loss of interest on capital that is required to keep a car for a year.
Generalised cost is the total cost to society of the trip. With public transport, the more users, the lower the generalised cost, but with cars the reverse is normally true. Car costs need to include a component for preventing old ladies crossing the road, preventing young children walking to school, and the pollution they cause, as well as delays. The most important point is that, in congested conditions, motorists do not pay the costs they impose on society, especially when buses are held up by queues of cars. The motorist’s marginal cost may be a only tiny fraction of the costs imposed on others.
Cost-benefit analysis has been used for many years by the Department of Transport to justify the construction of new roads, although some of the method is now out of favour. The costs are those involved in constructing the road and the benefits those resulting from a saving in journey time for those travelling on and near the new road. If the benefits exceed the costs, discounted over a number of years, the road was seen as worthwhile. Currently, rules prevent most public transport investments being justified on cost-benefit grounds alone. Instead, they must be ‘commercial’ although some ‘non-user’ benefits may be included.
In London, researchers using models and real data discovered that, in urban areas, a paradox existed because of the above rules. They discovered that
- a fixed amount of money invested in public transport produced more savings for the motorist than the same amount invested in roads.
A Cambridge parallel?
Some years ago I realised that most motorists saw cyclists as a hindrance to their progress and considered that a reduction in cyclists would speed up traffic. I considered the reverse to be true, as the one alternative for most cyclists is a car. To friends I proposed a ‘No Cycling’ day to prove my point, but (fortunately) no one took me seriously. The idea was to demonstrate to motorists that if all the cyclists who had cars used their cars to travel to work, congestion would be much increased. The logical extension of this argument is that if more motorists with cycles cycled to work, congestion would be much decreased. This suggests to me the hypothesis that
- Money invested in cycleways will produce a greater improvement for motorists than the same amount of money spent on roads.
How to quantify the case
Cambridge County Council has a traffic model called SATURN (Simulation and Assignment of Traffic in Urban Road Networks) produced by the Institute of Traffic Studies at the University of Leeds. When the correct data are loaded it can give good estimates of traffic flows and delays in Cambridge and its environs. It has in the past been used to predict the effect of road closures or new roads. My suggestion is that an assumption should be made that a proportion of cyclists desert their cycles and take to cars. Using this data, the model should be able to estimate the extra cost to the community of these decisions. This is a theoretical ‘No Cycling Day’. It should also be possible to calculate the effect of more motorists deserting their cars for cycles. These figures should make it possible to work out how much could be spent on cycleways to achieve say a significant increase in cyclists and still be positive in terms of cost-benefit.
A measure of the order of benefits available can be calculated from data in the Cambridge County Council’s Traffic Monitoring Report. Some 50 000 people travel into Cambridge at each peak on some 200 days per year and are each delayed by some 20 minutes. If time is valued at £6 per hour we have ‘available’ benefits of some £20 million per year. As has been shown above, unlocking say 70% of this would not require many people to switch from car to alternative modes.
There are difficulties
Firstly, how many motorists would cycle if improved cycleways were provided? The 1991 census showed that 66% of commuting journeys in Britain were less than five miles. Personal evidence suggests that a fair number of these under-5-mile trips could be captured, especially if the routes were as good as those in the Netherlands. There you can cycle two abreast and talk to a friend without the hazards of traffic fumes and erratic pedestrians.
Secondly there is a lack of good evidence of the actual journeys cyclists make. In 1987, an O&D (origin and destination) survey was done of a sample of cars crossing a cordon around Cambridge. Cyclists were not included, even though Cambridge is renowned world-wide for its cyclists. Although more recent observations in Cambridge have included volumes of cyclists, as far as I am aware there have been no O&D surveys. This makes SATURN’s predictions less certain.
My daily journey from a ‘necklace’ village to the centre of Cambridge is about 5 miles. Although there are cycle lanes (either white-lined or dual-use footways) for a significant part of the way, there are still difficulties:
- At the most hazardous points the cycleway disappears.
- Motorists obstruct the cycleway at peak times.
- Shared-use pavements are far from ideal for faster cyclists when there are side roads and significant flows of pedestrians.
- At peak times some 50% of my journey is past stationary or slow moving cars, buses, and lorries. The traffic fumes I am forced to inhale cannot be good for my health.
I believe that this is typical and that similar problems exist on most journeys where the choice is between cycle and car. Routes of this low standard do little to encourage people to cycle. But
- new and improved cycle routes will encourage motorists to cycle instead.
If these ideas were explained to the general public, I believe motorists, in particular, would be more sympathetic to expenditure on cycleways. Now that the Government has committed itself to increasing cycle use and reducing car use, we have the opportunity to test such ideas both in theory and practice. When Cambridgeshire County Council is spending more money on strengthening bridges for 40-tonne lorries than on cycle and pedestrian schemes it is clearly time to redirect resources.
- You do not have to force people out of their cars to make significant reductions in traffic, only provide suitable alternatives for people who would use them if they existed.
These arguments apply equally well to public transport investment, for instance in bus lanes, new bus routes or the St Ives railway line.