Safety and Mobility Impacts of Work Zone Lane and Shoulder Widths

The goal of this research was to quantify the mobility and safety impacts of different combinations of lane width and shy distance to the barrier for a given paved width in work zones. The research team developed a device to measure lateral distance and derive speed, vehicle length/type, and headway information under day and night conditions. Data were collected at 17 locations in Illinois, Michigan, and Wisconsin. Lateral distance data of over a quarter million vehicles were used for the safety analysis. Extreme value theory modeling was conducted to estimate the probabilities of right-hand edge line encroachment and right-hand barrier contact. Wider lanes were found to have decreased probabilities of edge line encroachment and barrier contact, while wider shy distances were associated with increased probability of edge line encroachment and decreased probability of barrier contact. The speeds of over 125,000 free flow vehicles were used to quantify the mobility impact. Linear regression was implemented to develop models for estimating free flow speeds in work zones. Work zone free flow speed increased with an increase in speed limit, lane width, and left-/right-hand shy distance to the barrier. A case study of a 55-mph posted work zone with two open lanes and barrier on both sides with 26-ft available paved width is presented. Results of the case study indicate that 11-ft lanes with 2-ft shy distance have a slightly lower probability of right-hand barrier contact (for vehicles in the right-hand lane) than 12-ft lanes with 1-ft shy distance, while having a greater free flow speed. This research has demonstrated how lateral distance can be collected and modeled along with speed data to assess safety and mobility impacts in work zones.