Comment on: Microscopic modeling of multi-lane highway traffic flow,   Nathan O. Hodas and Arnand Jagota, Am. J. Phys. 71 (12) 2003, pp. 1247
  In heavy traffic with congested roadway the maximum traffic flow also depends on length of cars. This is deduced in a simple derivation suited for classroom demonstration as well as homework. The resulting equation demonstrates a new relation to an apparently unrelated area of physics, the maximum ship velocity (hull speed) and explains why traffic is sometimes faster on the slow lane on a congested multi-lane road. 
M. Risch, (University of Applied Sciences, Augsburg, Germany)