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Cars & traffic

Oct 18, 2012

Is "Effective Speed" a useful concept?

"Effective Speed" relates travel time to hours worked and, at first glance, seems like an interesting perspective. Closer inspection suggests it's neither especially insightful nor particularly useful.

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Empty America - what San Francisco would look like without people

I might be late to the party on this one, but until I read this article on The Conversation this week, I hadn’t heard about the concept of “Effective Speed” before.

As explained by Dr Paul Tranter, a geographer at UNSW, Effective Speed takes into account all the time costs of a mode of transport, not just the in-vehicle time.

He says the time spent earning the money to pay for a car is usually much greater than the time actually spent driving. He cites this anecdote:

Imagine living in a village, where your job each day is to collect a bucket of water from the river. This takes an hour each day. To “save time” you build a machine to fetch the water. However, to make the machine work, you need to spend two hours each day winding up a spring.

In modern cities, he says, the equivalent of “winding up the spring” is the time spent at work earning the money to pay for transport. Driving to work might be considerably faster than using public transport or cycling,

However, motorists might be spending one or two hours per day (or more) earning the money to cover the cost of their cars, while cyclists spend only a few minutes per day earning the money to pay for their bicycles.

The term Effective Speed sounds very much like “Effective Density”, another relatively new and modish term. Effective Density combines job density and commute time in a single statistic.

I’m wary about the justification for combining a mere two ideas – both of them quite simple and straightforward – into a new and unfamiliar concept. Effective Density seems like an idea searching for a rationale (perhaps it’s about marketing more than anything else).

But at least Effective Density is conceptually logical. I’m not so sure the same can be said of Effective Speed. At first glance it’s appealing but on reflection it appears to be more illusion than logic.

It’s sensible to think about the full cost of transport (e.g. owning and operating a car) in terms of hours worked or money earned. But it’s not clear that comparing hours worked against hours spent driving adds anything or is even a useful way of thinking about transport.

To begin with, travel time leaves out a long list of other reasons people have for owning and operating a car. It’s not just speed – cars are attractive because they’re available on demand and go direct to the destination.

They also enable multiple passengers to be carried, as well as various objects like shopping or spare underwear. They offer privacy and personal security, protection from the weather and a high degree of comfort. Many drivers are also prepared to spend more (i.e. work longer hours) on a car  in order to communicate status.

Any metric that doesn’t factor in these attributes has limited explanatory power. My reservations go further though – what I don’t get is the premise that there’s a direct and special relationship between hours worked and hours spent driving.

Consider this example: I might work for six weeks to pay for a one week luxury holiday in the best hotel in Paris. The cost matters because I have to be able to afford it and I want value for money.

But how is evaluating the choice in terms of the length of the two time periods particularly important or illuminating? Is the trip not worth it because it’s much shorter than the time I have to work to pay for it? What matters is that I think the overall bundle of benefits exceeds the costs.

I suspect Effective Speed only seems appealing because both sides of the equation involve time. But if it’s a useful idea it ought to work for all the activities we labour to support.

What does it mean if I work one week a month to pay the mortgage? Or to pay school fees?  Or buy food? These are “effective”…….what exactly?

The limitations of Effective Speed are illustrated by this quote from Dr Tranter:

The higher trip speeds of cars do not save time; instead they encourage longer travel distances as the city spreads out and local shops, schools and services close.

Using travel time as the sole metric means the range of private benefits drivers get from their cars is ignored. The longer travel distances made possible by higher speeds mean drivers have a wider choice of destinations.

That might mean, for example, that they drive to better jobs, better dentists, better schools, or visit distant parents more frequently. Travel time is just a means to an end that, from the driver’s perspective, confers a private benefit and so is worth paying/working for (of course cars have social costs too, but that’s not what this discussion is about).

I’m not saying Effective Speed is “wrong”. I’m saying it doesn’t seem to be either particularly insightful or particularly useful.

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