http://support.sas.com/documentation/onlinedoc/stat/930/mcmc.pdf

I would not regard that as censoring because the variable "kilometres travelled" is naturally bounded at zero. By regarding this as some form of censoring mechanism one would assume that distance travelled could in principle be negative, but negative values were just not directly observed, for whatever reason. Maybe a GLMM with Gaussian or Poisson errors and a log link would be a more reasonable choice for this kind of data.