Myth: It's just as effective to promote carpooling

Myth: It's just as effective to promote carpooling as to provide public transport

Fact: In order to make carpooling effective, a majority of the target population must be convinced to change their behaviour. If the same number of people switched to public transport it would be vastly more efficient. And because carpooling is effectively an inferior kind of public transport, experience shows that genuinely good public transport is more attractive to people than any carpooling scheme.

It's sometimes believed that because people are supposedly attached to cars, a less 'painful' alternative to promoting public transport use is to encourage people to share rides in their cars. Some go as far as to suggest this can be as effective, or even more effective, compared to increasing public transport use.

Usually when carpooling is promoted, it is as a 'less worse' measure, so that people in car-dependent areas can give effect to their environmental aspirations until the authorities can be persuaded to improve public transport. Carpooling combines the worst aspects of cars with the worst aspects of public transport: it relies on people having similar origins and destinations and travelling at the same time; there is no network effect (where people can take advantage of intersecting routes to access more destinations); and there is an immense cooperative effort required. Public transport runs at the same time every day, but if a carpool driver is sick or alters their schedule one day, one has to make other plans.

So while there's nothing wrong with promoting carpooling as a sustainable transport alternative when public transport is inadequate and nothing can be done to improve it, it can be counterproductive to divert official resources to organising carpools that would be better used working for improved public transport.

(Of course, one should not confuse carpooling with car sharing - the various schemes that make shared cars available on a pay-as-you-go basis to people in inner-city areas. These schemes can help reduce car ownership levels, and effectively convert the fixed costs of driving to variable costs, although the benefits are offset by the additional car trips likely to be made by non-car-owners. But car sharing is largely confined to inner-urban niche markets, and even its promoters concede it is unlikely to work except as a supplement to good public transport and walkable urban design. Our problem is with those who promote carpools - where people still own just as many cars but attempt to share individual trips - as an alternative to improved public transport, particularly where public transport is poor.)

In practice very few carpooling schemes last very long before perishing from lack of interest or energy. This is because carpooling actually negates the reasons that draw people to cars in the first place. On sheer convenience grounds, carpooling is vastly inferior both to driving oneself and to using public transport where it exists. It also goes against the long-established trend in most developed countries, which is for car occupancy to decline, for 'car passenger' to decline as a mode of travel to work, and for vehicle-kilometres to increase faster than person-kilometres.

The very few carpooling success stories come, not surprisingly, from American cities where travel by car is relatively easy but public transport is of low to medium quality, poorly integrated, and designed to supplement car dependence rather than offer an alternative to cars. In such circumstances, carpoolers are as much or more likely to be former public transport users as former solo drivers. For example, a survey of the 'casual' carpooling that operates in the San Francisco Bay Area found that 75 per cent of carpool passengers, and 33 per cent of carpool drivers, were former train or bus users. If carpooling were not available, 90 per cent of those surveyed would use public transport; only 5 per cent would drive on their own. (The other 5 per cent would mostly use other carpool schemes.) The main reasons given for carpooling were to save time and to avoid high public transport fares. The report concludes:

It's not uncommon to shore up one myth by appealing to another, and that's what is being done here. Just as in Melbourne, an artificial capacity crisis has been used to excuse low public transport patronage in the Bay Area. At the time of this report, the BART had more tracks than the Toronto subway (for example), served a larger catchment area, and yet carried only one-third as many passengers. Perhaps the biggest problem with the RIDES report, as with other carpool promotion exercises, is the insinuation that while expansion of public transport services is difficult and expensive, road capacity for carpools is limitless and cheap.

But even in the US, the slight increase in carpooling in some US cities is just bucking an overall downward trend. US Census statistics show that between 2000 and 2005, carpooling as a share of journeys to work fell from 12.2 per cent to 10.7 per cent - even given the 50 per cent increase in petrol prices over the same period. Meanwhile in Melbourne, the proportion of people travelling to work as passengers in cars declined from 7.1 per cent to 5.6 per cent between 1996 and 2006. Indeed 'car passenger' is the only method of travel to work to show a consistent declining trend in recent times, both in absolute numbers and in overall share of travel.

Despite all the above, it is also sometimes suggested that carpooling is superior to public transport on energy-efficiency grounds (and, hence, in greenhouse terms as well). A typical argument runs as follows:

Leaving aside the fact that the premise is wrong - buses use about 3 times as much energy as cars, according to the National Greenhouse Gas Inventory and industry figures - the crucial unstated assumption here is that an increase in average car occupancy to two people is easily achieved, while it is much more difficult to improve bus patronage. In fact the reverse is true, as the Appendix below explains in detail. Increasing car occupancy requires an enormous effort in behaviour change, while increasing public transport patronage requires a comparatively small effort once improved services are in place.

Technical Appendix: How Many People Have to Carpool to Make It Worthwhile?

We argue in this Appendix that in order to make carpooling as efficient as the most mediocre public transport, the number of people who must change their behaviour is a clear majority of the travelling population. Yet if a similar number of people could be convinced to use public transport (a much easier task than convincing this number of people to carpool) the overall efficiency would be vastly greater.

As a 'benchmark' for mediocre public transport, consider buses in Melbourne. In Melbourne, use of buses runs at about 3 per cent of all trips. Perhaps ironically, this very low usage means that impressive gains can be made by convincing a relatively small number of people to shift to buses. To double bus patronage, for example, means changing the behaviour of around 3 per cent of the target population, or one person in 33. Experience in cities that have improved their bus services shows that this is readily achievable through measures such as increased frequencies, traffic priority, and easy-to-understand routes.

This points to the first problem with comparing the energy use in a carpool to that in a bus that is so poorly utilised it only carries an average of 10 passengers. If it is being seriously proposed that transport policy favour carpools over both single-occupant cars and public transport, one has to compare the hypothetical scenario where a sizeable number of people carpool to that where a similar number use public transport. But when public transport starts to carry large numbers of people (say 20 per cent of the population), economies of scale kick in and fewer vehicles are required to carry a given number of people. Cities with this rate of public transport use have occupancies significantly higher than 10 per vehicle. Even if average bus occupancy is 20 per vehicle rather than 10, the carpool rate to achieve the same energy efficiency is around 6 per car - virtually impossible to achieve in practice.

So, carpooling can only really be put forward as an energy-efficient transport measure if one has already given up on making public transport compete with the car. But, one may say, suppose it's easier to convince people to carpool than to carry out the planning and investment overhaul needed to make public transport work? Then one can at least improve energy efficiency for less effort, even if the result is less impressive than achieved in other cities.

However, this 'carpooling as easy second-rate option' relies on the untested claim that it's easy to convince large numbers of motorists to carpool instead of driving themselves. This is the second problem with the naive comparison: it puts forward a number (2 per vehicle) without examining the actual behaviour-change effort it represents.

So, how many people need to change their behaviour in order to increase average car occupancy to 2 per vehicle? If everyone currently drives alone, and the target is to get people to travel in pairs, you have to change everyone's behaviour! Half the drivers must become passengers, and the other half must adjust their travel plans to carry a passenger.

But of course this is only the worst-case scenario. Most cars can carry more than 2 people: surely if you can get a proportion of people to travel in groups of 4 or more, then you can get the average up to 2 per vehicle without everyone having to participate. Indeed (it's thought), only a minority of people have to share in larger groups to double the average occupancy.

This sounds good until you do the maths. Assume that currently there is only one person to each car (this is not too far from the truth). Suppose that out of a target population of N people, a number P start carpooling with 4 to a vehicle, and the remainder (N - P) continue to drive alone. The P carpoolers occupy P/4 cars, while the sole drivers occupy N - P cars. The total number of cars is then N - 3P/4, and the number of travellers is N. To get the average car occupancy, divide the number of people by the number of cars to get N / (N - 3P/4), or (after cancelling), 1 / (1 - 3P/4N).

For the average occupancy to be 2 per car, we need 1 / (1 - 3P/4N) = 2, or 1 - 3P/4N = 1/2, or P/N = 2/3. In other words, 2 out of every 3 people must carpool in order to get the average occupancy up to 2.

So the behaviour change required is still a pretty mean feat: while we don't have to get everyone to change, we still have to convince a clear majority to do so. And even then, we only get the desirable result if all the people who carpool manage to organise themselves in groups of 4.

But, one may still protest, why limit the carpool size to 4? Suppose we really get serious about this, get everyone with a van or minibus to join the pool, and get really large groups into each carpool vehicle. Then, surely, we can get average occupancy up to 2 per vehicle without having to convince more than half the population to join in?

Sadly, no. Consider the extreme case, where the P carpoolers all manage to squeeze into just one vehicle. With (N - P) single-occupant cars and one huge carpool vehicle, we have N people in N - P + 1 cars, for an average occupancy of N / (N - P + 1) = 1 / (1 - (P - 1)/N). Setting this equal to 2 gives 1 - (P - 1)/N = 1/2, or P = N/2 + 1. So even in this extreme situation, we need to convince half the population (plus one) to participate. In the real world, where more than one carpool vehicle will be required, the number P must be quite a bit larger than N/2.

There is still one objection remaining. We know that currently, not every car on the road has just one person on it. Average car occupancy for journeys to work is somewhat higher than 1, actually about 1.15. So we've at least got a head start in raising occupancy to 2.

While this is true, it doesn't really alter the conclusions. The difference between 1 and 1.15 is 15 per cent, and half the population less 15 per cent is still pretty close to half the population. A numerical example will help make clear that there is still a long way to go.

Consider a 'realistic' breakdown of 1,000 cars selected at random, according to number of passengers. One such breakdown, with an average occupancy of 1.16, is

What proportion of travellers are sole drivers in this example? The answer is 900 divided by 1,160, or 78 per cent. Similarly 10% of travellers are in groups of 2, 5% in groups of 3, and 7% in groups of 4.

Most trips with passengers now are shopping or leisure trips that are difficult to convert to carpooling. So let us focus on those 78% of people who travel as sole drivers, and suppose we get some of them to convert to carpooling in groups of 4. To reach an average car occupancy of 2, we must reduce the number of cars from 1,000 to 580 (half of 1,160). In other words, we must take single-occupant cars off the road and replace them with cars containing 4 people, so that the net number of cars taken off the road is 420. It turns out that to do this, we must take 560 sole drivers and put them into 140 cars, so that the breakdown of cars and travellers becomes as follows:

Thus to achieve an average car occupancy of 2 under 'realistic' conditions, we had to convince 560 people out of 1,160 to switch from driving alone to carpooling. 560 out of 1,160 is 48 per cent: still close to half the population.

What would it take to get the same energy saving with public transport? Again we can look just at buses, and contrast the Melbourne situation with a 'best practice' benchmark such as Toronto in the early 1990s. Of our population of 1,160 people in Melbourne at present, we could expect about 35 (3 per cent) to be bus users, and to occupy 4 buses with an average occupancy slightly under 10. These buses typically run at half-hourly or hourly frequency. The overall effect is to take about 23 cars off the road, assuming bus passengers would otherwise be solo drivers.

In a "world's best practice" city, on the other hand, buses run every 5 to 10 minutes and have an average occupancy of 20 to 40 people. Thus the equivalent population would have 24 buses available rather than 4, with minimum expected patronage of 480 passengers. Each additional bus consumes energy equal to about 3 cars, so if all the bus passengers were former sole drivers, the energy saving is equivalent to taking at least 420 cars off the road, the same as achieved in our hypothetical carpooling example. But note the following crucial differences:

The moral of the story is that honest comparisons of relative energy efficiency compare achievable levels of public transport patronage (according to world's best practice) with the level of car occupancy achievable with the same expenditure of money and effort. When you do this, public transport is the clear winner on efficiency grounds, as ordinary common sense suggests.

© 2007 Public Transport Users Association Inc. (PTUA), Victoria, Australia. ABN 83 801 487 611.
General copying and distribution on a non-commercial basis is permitted subject to proper acknowlegement.
Authorised by Tony Morton, 247 Flinders Lane, Melbourne, for the PTUA

Common Urban Myths About Transport

Technical Appendix: How Many People Have to Carpool to Make It Worthwhile?