Mode bias in traffic forecasting

Recently released 2009 National Household Travel Survey from the Federal Highway Administration documents transportation mode choice trends. From 2001 to 2009, the percentage of biking, walking, and transit trips have increased while vehicle trips have fallen.

According to the Mobilizing the Region web site:

Of course, the good news for walking, bicycling and transit use may reflect fluctuating gasoline prices and the current economic recession (respondents were surveyed between March 2008 and April 2009). But when asked to interpret the findings, FHWA staff told MTR that “the general public is more aware of the need to walk and bike for environmental and health reasons, thus they make more of those trips and they report more of them in our survey.”

What we have found in Metro Detroit is traffic engineers assume there will be increased vehicle traffic, even in cities which have undergone drastic populations drops like Royal Oak and Detroit. (Note that according to U.S. Census data, Royal Oak’s population decline rate is slightly higher than the city of Detroit’s since 2000.)

Assuming increased vehicle travel often means our roads are wider than they need to be — width that could be converted into bike lanes.

Washington Boulevard in Royal Oak

For example. last year we tried pushing the city of Royal Oak to change Washington Boulevard before it was repaved between Lincoln and Woodward. We asked for a 4 lanes to 3 road diet which would have added bike lanes while increasing safety for all users. We were told that the city was projecting an increase in vehicle travel on Washington over 20 years and therefore could not reduce the number of lanes.

This projected increased was standard practice but completely unrealistic given that:

  • Washington Avenue north of Lincoln had already been road dieted from 4 to 3 lanes.
  • Woodward vehicle traffic was declining.
  • Royal Oak’s population is dropping.

(Of course what’s ridiculous is that the road diet could have occurred for the time being. If that vehicle traffic did return, the road could be re-striped. In the meantime, there would be increased safety for all travel modes. Clearly safety was not the highest priority in this decision making process.)

I-94 Expansion in Detroit

Another example is the planned expansion on I-94 through Detroit. MDOT is still forecasting increased vehicle traffic even though the actual numbers show no increase.

On the other hand, MDOT decided to remove two pedestrian bridges after concluding they weren’t justified given the existing pedestrian and bicycle traffic. MDOT did not project any growth for biking and walking on these bridges despite the US DOT policy statement that such projects “should anticipate likely future demand for bicycling and walking facilities.”

Their bias is apparent.

We need to ensure that Metro Detroit transportation projects realistically and consistently forecast traffic for all modes.

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