Induced Demand and Road Diet

Recently I read the book ‘Walkable City’ from Jeff Speck. The book discusses in 10 steps how we can make cities better, more attractive and healthier. As always when I read an urban planning book, I wonder how this would translate to Dartmouth. Regrettably, most great books are about city planning and, when we’re lucky, about town planning but hardly ever about campus planning. Nevertheless, I am going to try to translate Speck’s lessons for a walkable city and ask what this would look like for Dartmouth.
Speck’s most eye-catching quote is: ‘Traffic Studies are Bullshit’. Besides the fact that I like the controversial language, I agree with his opinion that the car-first approach has been damaging to American cities. Any significant change to a neighborhood requires a traffic study, any new street design needs a traffic study and guess what: they almost always predict an increase in traffic. The traffic studies are almost always right as well. The reason for this is ‘induced demand’. ‘On average, a 10 percent increase in lane miles induces an immediate 4 percent increase in vehicles traveled on that road, which climbs up to 10 percent in 10 years. In other words, the induced demand is so large that eliminates all the advantages of adding lanes.


Now, we do not live in a metropolitan area and many people are completely dependent on the car to get to work, but does creating more roads solve the congestion issue around Dartmouth?
With a ‘road diet’, Speck argues that decreasing the amount and the width of lanes can substantially increase the walkability and dynamic of a city without creating more congestion. In fact, he argues that narrower lanes are safer. Fewer fatal accidents happen, and cars drive substantially slower. Chances of a pedestrian surviving a traffic collision would be 35% higher if collision speed were reduced from 30mph to 20mph. The chances would be 85% higher if the speed were reduced from 40mph.
Where could this work for Dartmouth? Two places immediately come to mind. One is the traffic around the Green, the other is the access roads. Roads around the Green can be identified as three one-way roads and one two-way two-lane road with a safety island in the middle, street parking on one side and a bus-stop lane on the other side.  From a quick satellite picture measurement, the three one-way roads are approximately 26′ (Dartmouth Hall Side ), 24′ (Baker Library Side), and 33′ (Collis Side) wide.  The width of each of those lanes on those roads are at least the same width of highway lanes which encourages people to drive fast, especially if there is no oncoming traffic.
The access roads are the Wheelock Streets (East and West), Lyme Road, Route 120 and Route 10. The speed limit of those roads in town is 25mph. These roads also have at least 12 foot wide lanes, again the width of highway lanes. Though direct oncoming traffic and street parking reduce the ‘sense of safety’, the width of the lanes still encourage speeding.

I understand that snow removal may require some additional space in the lanes but it would interesting to take a close look at some of those roads and see if narrowing lanes, adding bike paths and return to 2-way streets around the Green could significantly improve the traffic situation, including bicycle and pedestrian friendliness and the livability of our Campus.

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