Thursday, March 14, 2013

Boston MPO memo fails to understand purpose of HOV lanes


The Boston MPO recently released a memo titled Screening Regional Express Highways for Possible Preferential Lane Implementation. Quote:
The preferential lanes envisioned in this study would usually constitute specialized, separate lanes, generally constructed in the median of an expressway. The adjacent main travel lanes could continue to accommodate their full lane capacities, up to around 2,200 vehicles per hour, depending on the design standards of the roadway. With preferential lane eligibility set to allow fewer vehicles, perhaps 1,500 per hour, buses and other users of the preferential lane would be able to travel at posted speeds. Depending on traffic conditions, users of the general-purpose lanes would continue to experience delays either as a result of heavy traffic or from queues forming at bottlenecks.
It is sometimes suggested that an HOV lane could be implemented by converting a general-purpose lane for the exclusive use of HOVs. Since it is assumed that any preferential lane eligibility rules would result in fewer vehicles in the preferential lane than in the general-purpose lanes, the result would be a reduction in total expressway capacity. Reducing the capacity of a congested expressway would seriously worsen congestion and queuing within and leading to the capacity-reduced corridor, as well as on nearby surface roadways.
Here's the author's reasoning:

  • General purpose lanes have a capacity of 2,200 vehicles per hour
  • HOV/preferential lanes should be limited to 1,500 vehicles per hour
  • 1,500 is less than 2,200
  • Therefore, general-purpose lanes should never be converted to HOV lanes.
Have you spotted the flaw yet? It's the measurement unit: vehicles per hour. The author considers that all vehicles are equal with regards to congestion. Therefore a single-occupancy car is equal to a four-person carpool, and both are considered equal to a bus carrying 50 people. This memo blithely ignores the entire purpose of HOV lanes -- which is to expedite the movement of people, not pieces of metal.

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