Friday, August 03, 2007

Ten Million Daily Crossings Over Deficient Bridges

Both the Boston Globe and the Boston Herald both have front-page reports today on the state of Massachusetts' bridges, particularly the 588 of them that the Federal Highway Administration has deemed "structurally deficient", the same designation that the I-35W bridge over the Mississippi river had before it's tragic collapse on Wednesday. The Globe has a list of the top 100 deficient bridges as well as a map. Still, I wanted to look at the whole dataset (why stop at 100, after all?) and went to the FHWA's National Bridge Inventory and downloaded the Massachusetts dataset myself. Here are a couple of interesting (read terrifying) facts about those 588 bridges that the FHWA has classified as "structurally deficient":

  • Massachusetts drivers make an average of 10 million crossings daily over structurally deficient bridges.
  • Laid end-to-end, the span of these bridges would be just over seven miles.
  • The top six most heavily travelled of the bridges are used by Rt. 128, in Canton, Dedham, Newton and Wakefield
  • Forty-seven of the bridges are in the City of Boston. Fall River is a distant second with 15.
  • The majority of the bridges cross railroad tracks or another highway.
  • Fifteen of the bridges over the Charles River are structurally deficient. The next highest total by waterway is the Sudbury River with twelve.
  • The road with the most structurally deficient bridges is Route 2, with 19. The next highest is a tie between the Mass Pike and I-91 with 14 each.
  • The total estimated cost to repair or replace these bridges is over $2 billion in year-of-estimate dollars. Cities and towns are responsible for nearly $400 million of that total.
  • While this article claims that the Bourne Bridge is the most highly trafficked structurally deficient bridge in Massachusetts, the FHWA data shows that bridge as being only "Functionally Obsolete". Here's the difference:
A bridge is Structurally Deficient if it is in relatively poor condition, or has insufficient load-carrying capacity. The insufficient load capacity could be due to the original design of an older bridge that used lighter design loads, or due to deterioration. A bridge is considered Functionally Obsolete if it is narrow, has inadequate under-clearances, has insufficient load-carrying capacity, is poorly aligned with the roadway, and can no longer adequately service today's traffic.
Now, I should point out that the average daily crossings and total project cost numbers are given by the FHWA only for the year the bridge was last observed. That means I am somewhat inappropriately adding up trips and dollars from anywhere between 1996 and 2006. While normalizing these to get current-year costs and current-year trips would give us a better picture, the figures I've presented without doing so are good-enough for back of the envelope calculations.

Here are the top-ten communities in terms of average daily traffic over structurally deficient bridges:
CityTotal BridgesAverage Daily TripsCombined Miles
Boston 47 1,449,187 0.73
Dedham 5 455,000 0.04
Newton 8 384,317 0.13
Canton 3 321,500 0.04
Fall River 15 308,100 0.44
Danvers 5 267,610 0.03
Framingham 10 203,750 0.12
Attleboro 4 186,300 0.04
Northampton 12 185,813 0.14
Springfield 6 181,095 0.09