How does the brain calculate velocity?
Motion perception

This article on motion perception might be a good start.


  pure motion perception is referred to as "first-order" motion
  perception and is mediated by relatively simple "motion sensors" in
  the visual system, that have evolved to detect a change in luminance
  at one point on the retina and correlate it with a change in luminance
  at a neighbouring point on the retina after a short delay


Vehicle velocity

There are a few articles on estimating vehicle velocity.
Scialfa et al (1991) provides a more behavioural account focussing on age differences. 

References

Scialfa, C. T., Guzy, L. T., Leibowitz, H. W., Garvey, P. M., &amp; Tyrrell, R. A. (1991). Age differences in estimating vehicle velocity. Psychology and aging, 6(1), 60. PDF