I-66. Photo by thisisbossi on Flickr.

Lengthening all Metrorail trains to eight cars long would add as much capacity to the I-66 corridor as widening the highway by two to four lanes.

If Metro lengthened all trains to eight railcars, it would increase capacity on the Orange/Silver Line through Arlington by 4,740 passengers per hour per direction, according to WMATA’s PlanItMetro blog. Comparatively, one new highway lane would be able to carry 2,200 cars per hour.

Even assuming two passengers per car (likely higher than the real average), a new highway lane would only carry 4,400 passengers per hour. Still fewer than 8-car Metro trains.

Then, to account for the reverse direction, double all calculations. Bidirectional Metrorail capacity would increase by 9,480 passengers per hour, equivalent to 4.3 lanes full of single-occupant cars, or 2.15 lanes full of cars with two passengers each.

Eight-car trains would also be cheaper and carry passengers faster than equivalent new highway capacity, PlanItMetro notes.

Clearly it’s time to think longer, not wider.