Extraordinary Facts from the Past: How traffic projections were done

As shown in a previous post, it turns out that Downtown Vancouver traffic volumes in 2010 are equal to those in about 1965.  But you can bet that the traffic projections done when the original graph was drawn in 1976 would have projected a massive increase in traffic by now.

How would traffic planners have done that?  Most likely by drawing a straight line based on past results, as Sightline Institute intern Alex Broner discovered when he came across an old report.  Clark Williams-Derry explains in Traffic Forecasting: A Blast from the Past:

.. the safest way to predict the future (was) to cherry-pick a reasonably stable period of growth in the recent past, and simply assume that traffic will continue to grow that fast in the future.

As simplistic as this technique seems now, it probably worked just fine for decades! Back when gas was cheap, median incomes were rising, and the baby boomers were moving towards their peak driving years, traffic volumes really did increase steadily. The only interruptions were temporary: the occasional recession or oil price spike. …

And that’s how we wind up with traffic forecasts that are completely at odds with the reality of the last decade. Forecasts of ever-increasing traffic used to be standard practice. But now, as many of today’s engineers are figuring out, the future doesn’t always look like a straight-line projection from the past.

Charles Marohn, in his latest post, illustrates where that kind of thinking takes us: in massively overscaled, expensive and underproductive investment in more road infrastructure – in this case,freeway ramps in Chester, Pennsylvania:

These ramps represent an investment of $9,060 for a Chester family of four. For a city with a 12% reported rate of unemployment, how many real jobs could be created if $77 million were put into an economic gardening program? This is an unconscionable expenditure.