If the optimal overall speed is determined by greatest efficiency, which means a reasonable maximum speed, then I would think traffic would have an even bigger

If the optimal overall speed is determined by greatest efficiency, which means a reasonable maximum speed, then I would think traffic would have an even bigger impact: it's not profitable to compensate for heavy traffic with absurd speeds in rural highways, so the overall speed is really just determined by (1) amount of traffic, (2) battery size, (3) charge time, and (4) aerodynamics. In this case I'd expect traffic is really the dominant factor.