Republished from Newton City Coucilor Jake Auchincloss’ latest newsletter.
We tend to think of traffic like a liquid: it requires a certain amount of volume to flow smoothly. Congestion, in this view, is the result of constrained volume. Widening roads should reduce congestion, because automobiles will flow through a wider aperture.
This does not work. Widening roads does not improve congestion. Expanding capacity encourages more people to drive. Economists and urban planners call this induced demand .
The better analogy, then, is to think of traffic like a gas: it expands to fill whatever volume it is in. Congestion, in this view, is not the result of constrained volume but of expanded supply. Automobile capacity, whether in the form of lanes or parking spots, is a good subject to supply and demand. Expand the supply of automobile capacity and you get a higher quantity of demand.

Adding lanes & parking induces more trafficAdding lanes & parking induces more traffic

Wider roads? More drivers. More parking? More drivers. Three important corollaries:

  1. Reducing vehicle lanes of travel does not have to mean more traffic. Indeed, preventing lane-switching removes a principal source of back-ups. For this reason, I am receptive to Washington Street proposals that would reduce automobile lanes from four to three to accommodate less congestive travel modes.
  2. Adding parking spots does mean more traffic. Consider the propose Northland development on Needham Street, which envisions 800+ housing units and 400K+ sf of office and retail. They want 1,900 parking spots. I have said repeatedly I will not vote for that. 1,900 parking spots would induce significant traffic onto Needham Street. Northland’s transportation demand management program must radically shift away from parking and driving.
  3. If expanding the quantity of lanes and parking does not reduce traffic, then price is the only other lever. Allowing the price of driving and parking to adjust according to supply and demand will ‘clear the market’ of traffic.
When prices of driving and parking can adjust, traffic clears

When prices of driving and parking can adjust, traffic clears


Governor Charlie Baker has, so far, rejected congestion pricing for driving in Massachusetts. Here in Newton, though, City Councilor Andreae Downs and I are working to pilot ‘differential pricing’ for parking. The city could raise parking rates in areas where there is more demand than supply, like retail storefronts in Newton Centre, and lower them in underused zones. Shoppers who prize convenience may park closer to commerce; employees and commuters who seek long-term parking at lower rates may park farther away. When applied elsewhere, parking has become easier to find and there is less traffic from circling for spots. (You can google Donald Shoup + parking for a full review of policies.)
Finally, a caveat. I wrote above that traffic operates like a gas, not a liquid. As with all analogies, this one has its limits. Traffic’s liquid-like flow is, indeed, an important consideration. Syncing traffic lights, for example, can reduce congestion. Bottlenecks like exit 17 would benefit from higher throughput. The point is not to disregard traffic engineering—which we all experience and opine on—but to emphasize traffic economics, which is disregarded but more consequential. 

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