Sometimes a longue durée might be a better guide. We see this especially as we look at the post-war distortions created by cheap oil. Eisenhower America is over, but you'd never know it by standard Planning doctrine.
The orthodoxy is visible in projects large and small. Reading the supplemental materials for the Bone Parcel in December, I found a set of assumptions in the consultant's Transportation Impact Study that look not just exceptionally robust, but even crazy.
For all of the intersections but one, with the City's blessing the consultant assumed an annual growth rate of 5% in car traffic.
Over the 20 year horizon, a 5% rate represents an increase of 250% over the current car traffic volumes!
Driving has Plateaued
But the real world just doesn't support this. In a comment yesterday, Curt pointed out a tunnel in Brisbane Australia whose traffic modeling was fatally flawed with wildly overoptimistic growth projections.
(Graph: Impresa Consulting)
But it's not just in Australia. Even before the Great Recession started, vehicle miles traveled had plateaued here in Oregon. Absolute trip numbers aren't increasing either. We know gas will not get cheaper, and we're pretty sure energy in general will get more expensive. Moreover, the decline in driving among younger people is even more dramatic than declines among older cohorts.
Here's the traffic over the Marion and Center Street Bridges. This is not VMT, but absolute counts. It shows the same pattern: A dip circa 1985, a small excess in the 90s, and a much larger decline off the trend line in the 00s. It's plateaued as well.
Since the gas tax is broken (think electric cars!) as a source of highway funding, we're also going to see new financial incentives to drive less: In addition to seeing some kind of carbon tax, we'll probably also see a tax on mileage, on VMT. Not to mention tolls and congestion pricing. There will be many incentives to be more choosy about driving a car!
Here's a revised chart of drivers licenses in Marion County that really shows the start of a generational shift in attitudes towards auto-oriented mobility.
So even though this plateau is also recent, the best available information suggests not that driving will continue to increase at 5% a year, but that very real structural brakes on driving increases are looming on the horizon.
It's just not likely that car driving will continue to increase at a 5% rate!
But that's what we plan and spend for. No wonder things are messed up!