Empirical Analysis of Vehicle Time Headways and Speeds on Rural Two-Lane, Two-Way Roads

Abstract

Accurate knowledge of vehicle time headway (TH) and speed distributions is essential in many traffic engineering applications, such as capacity and level-of-service analysis, and in several contexts (operations at road segments, priority junctions, roundabouts, merging maneuvers). Other fields of interest are represented by vehicle generation in traffic microsimulation models and driving simulation applications. This paper presents results from an experimental analysis of vehicle TH and speed distributions on two-lane, two-way roads. The focus is on rural roads in Northern Italy, which are characterized by various levels of traffic and where data were collected by radar sensors. A trendless analysis of observed THs was carried out. Analysis then focused on estimating a well-known TH model, gamma-generalized queuing. Statistical analysis of the available data allowed testing of a set of speed distribution models, highlighting their goodness of fit with reference to empirical distributions. Completing the analysis, bivariate distributions were estimated to represent headway and speed distributions. The traffic parameters considered to affect TH distributions were flow rate and flow composition (percentage of heavy vehicles). Exogenous conditions, such as weather and road geometric futures, were common to all analyzed time periods and cross sections. The final goal of the analysis was to create a picture of the relationship between traffic conditions and speed and headway distributions for a typical road.