Unified approach to estimating free speed distributions

We present a new approach to estimating the distribution of free speeds based on the method of censored observations. The original distribution-free method of Kaplan-Meier is generalized to include partially censored data, i.e. observations that are censored with a certain probability. This is achieved using a composite time headway distribution model that is estimated as well. Using synthetic data, the method is validated. The method is applied using real life data collected at different two-lane rural roads in the Netherlands to establish free speed distributions and the differences between different vehicle-types.