November 28, 1998


Originally published in issue 33 of Tollroads Newsletter, which came out in Nov 1998.


Subjects:STPP distortion of statistics


STPP took TTI’s 70 metro areas ranked them by growth in lane capacity, divided them into two groups the 35 areas that built over-average new road capacity and those that built under-average new capacity. The top group of cities added 47% to its lane-miles over 15 years, the bottom group only 22%. STPP took the various measures of congestion and, lo and behold, there is little difference between the two groups. Therefore says STPP roadbuilding doesn’t help. Doesn’t follow.

American cities have grown at hugely different rates in the past 15 years. New York, Pittsburgh, New Orleans, Boston and Milwaukee have grown by single digit percentages. By contrast Atlanta, Orlando, Austin, Las Vegas, Phoenix have grown by over 50% in the 15 years. In the high growth areas they have built more new highway because the new communities demanded major road improvements. What official would be re-elected who suggested they keep little 20 foot farm roads as their primary roads in the face of development?

The bigger roadbuilding cities have tended to be those coping with larger populations and consequent heavier traffic. They have been fighting to keep up with more rapidly growing traffic, so it is not surprising that on average they have been no more successful than other cities in coping with congestion. All the STPP’s crude numbers demonstrate is that there is no systematic difference in the tolerance for congestion between fast-growing and slower-growing cities.

To assess if roadbuilding alleviates congestion, compared to not building new road capacity, you need to take these very different rates of population growth into account. When you discount roadbuilding by population growth you get an entirely different picture from that presented by STPP. I did a spreadsheet (Excel) which estimated extra lane-km/population growth. Ranking the cities by the strength of their roadbuilding relative to their population increase, then dividing them in halves as STPP did, the strong roadbuilders, the top half, had an average increase in congestion of 22%, whereas the weak roadbuilders, the bottom half, suffered a 31% average increase in congestion.

Similarly if you discount roadbuilding efforts for the growth of vehicle-miles traveled (VMT) — a measure of demand for roadspace — in each metro area you get a similar result. The more energetic roadbuilding cities had a 21% increase in congestion compared to a 32% increase for the less energetic ones.

In other words the relatively weak roadbuilding cities suffered about a 50 percent greater rate of increase of congestion than the relatively strong roadbuilders. As the TTI report itself says: “One solution to the congestion problem is addition road construction.” (pxii) This is commonsense of course, but STPP seems determined to inject nonsense into the public policy debate.

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