We currently use a Poisson process model, but, you should not forget that this approach is not very realistic because a Poisson process stands for no-related events and obviously the time of occurrence of a car passby is related to many factors on the nexts and previous cars. More sophisticated models are available such as microsimulation traffic models that you can also buy implemented as dedicated software.
I think this links can talk by themselves:http://www.traffic-simulation.de/
"The influence of traffic flow dynamics on urban soundscapes" from Decoensel et al. Applied Acoustics 66 (2005) pp. 175–194
Hope it helps.
CONICET PhD Fellow at
Acoustics and Electroacoustics Laboratory
National University of Rosario