Saturday, October 11, 2014

The carpool lane

One of the big incentives for most electric vehicle customers in the San Francisco Bay Area is the access to the diamond/carpool lane.  For commuters that have inflexible work hours, driving in the carpool lane can cut ten minutes or more from your trip.  Because of the hours I tend to drive, the carpool lane is seldom an option for me.  Along my route to work, the carpool lane is restricted only during the morning and afternoon peak hours, and I tend to commute just after each of these restricted periods.  So, most of the time, the carpool lane is not a viable option.  Of course, the carpool lane is not without its costs either.

Like any car, an electric car uses the least amount of energy when a steady speed is maintained.  Although stops can recapture up to one-third of the energy used to accelerate, it cannot recapture all of the energy, nor do you recapture the steady-state energy.  The trouble with driving the carpool lanes is the unpredictability of the flow of traffic.  At any moment, a car ahead of you may need to slow in order to change lanes, or another slower car can pull in front of you from an adjacent lane.  Finally, getting into the carpool lane often requires harder acceleration than simply pulling onto the freeway (if you want to avoid a rear collision).  So, on those few trips I take when the carpool lane is available, I tend to evaluate the flow of traffic outside the carpool land and decide whether traffic is moving steadily or stopping and starting.  I will stay in a steadily moving lane as long as the speed is at least 30 MPH and elect to use the carpool lane when it drops below that point.  (Besides, driving at these slower speeds uses substantially less energy.)

So, if I seldom use the carpool lane, even when it is available to me, why did I bother applying for the special decals allowing me to do so?  The other diamond lane available to me is found at metered freeway onramps, where a traffic signal controls which cars enter the freeway and how often.  At some of these metered entrances, a diamond lane has been set up affording carpoolers a shorter wait to get into traffic.  These express ramps can save as much as three to six minutes time getting into traffic on the freeway, and I take advantage of them every chance I get.

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