We never add capacity to our roads on the basis that traffic will continue to grow at historic growth rates #alternativefact
So if you missed the whole #alternativefact thing, you must have been boycotting all Trump news this past weekend. I couldn't get enough of the train wreck so if you did miss it, apparently, it's the cool new way to claim that things you clearly know are false are actually true!
This is a pretty big issue for any transportation engineer these days. Historically the impact of many new developments has been calculated on the basis of new vehicle trips estimated from ITE (Institute of Transportation Engineers) trip rates, which themselves are based on historic trips surveyed at specific land uses. These rates are then applied to the gross floor area or number of units to determine the number of new vehicle trips for a development. The new vehicle trips are added to the existing traffic to determine traffic volumes at opening. Seems reasonable... but after that, background traffic growth will be added to determine the horizon year traffic volumes so that the necessary number of vehicle lanes can be determined to accommodate all that "predicted" traffic 15+ years from now.
Oops! We just made it easier to drive and therefore induced traffic. We also just widened an intersection or a street on the basis of what might happen. That was a reasonable assumption 20 years ago, but as well as realizing the cost of this to the urban environment and general health and well being of our cities, we are entering the first period of great uncertainty for the automobile.
The problem is, none of us really know how vehicle use is going to change, we're just pretty sure it will. That's good enough reason not to be building any extravagant vehicle infrastructure at this time.
And that's not to say we shouldn't build any, there are many reasons we still need to build, rectifying design mistakes from the past, improving safety, replacing infrastructure that is coming to the end of its life, facilitating goods movement, etc. Each situation is different, its just important that we judge each on the basis that past growth is not necessarily a good indicator of future demand.