title: Exploring the impact of access designs on crash injury severity on multilane highways.
abstract: OBJECTIVE
  Access design is a critical factor that influences the safety and mobility of urban/suburban multilane highways due to the interactions between access movements and through traffic. An effective way for improving the safety and mobility of multilane highways is to control access maneuvers by implementing appropriate access designs. Understanding the impact of access designs on crash injury severity is beneficial for implementing effective countermeasures to mitigate crash injury at access points. Thus, the objectives of this article are to investigate the impact of access designs on crash injury severity and identify contributing factors of crash injury severity at access points of multilane highways.
  
  
  METHODS
  A total of 1830 crash records were collected at 149 access points with different access designs for a period of 3 years (2008-2010) in Florida. A heterogeneous logit model, relaxing the constraint of identical variances across observations in the traditional ordered choice models, was developed to evaluate the impact of access designs on crash injury severity and identify contributing factors. The marginal effects of the developed model were used to interpret the impact of access designs and other contributing factors.
  
  
  RESULTS
  At 4-leg access points, given that a crash has occurred, replacing full median openings with closed medians will reduce the probability of severe injury (fatality, incapacitating injury, or nonincapacitating injury) by 9.73 percent; substituting full median openings with directional median openings will reduce the probability of severe injury by 11.02 percent. At 3-leg access points, given that a crash has occurred, closed medians significantly experience a lower risk of severe injury than full median openings; however, there is no evidence that directional median openings are similarly effective. Other contributing factors of crash injury severity at access points were identified as number of lanes, shoulder width, median width, driveway density, left-turn bay on major roads, speed limit, average annual daily traffic (AADT), high-density residential area, daylight, driver age, and truck involvement.
  
  
  CONCLUSIONS
  Installation of directional median openings is a reasonable safety treatment at 4-leg access points because this access design has better safety performance than full median openings in terms of crash injury severity; in addition, it provides fewer restrictions on accessibility than closed medians. Other effective treatments include installing left-turn storage bays at median openings and increasing the width of shoulders and medians. Supplemental materials are available for this article. Go to the publisher's online edition of Traffic Injury Prevention to view the supplemental file.
authors.affiliation: a University of South Florida , Tampa , Florida.
authors.forename: Zhenyu
authors.initials: Z
authors.lastname: Wang
authors.forename: Bing
authors.initials: B
authors.lastname: Huang
authors.forename: Jian
authors.initials: J
authors.lastname: Lu
authors.forename: Jianyou
authors.initials: J
authors.lastname: Zhao
authors.forename: Yu
authors.initials: Y
authors.lastname: Zhang
mesh_terms: D000063:Accidents, Traffic / Q000706:statistics & numerical data*; D004779:Environment Design / Q000706:statistics & numerical data*; D005431:Florida / Q000453:epidemiology; D006801:Humans; D015233:Models, Statistical; D012307:Risk Factors; D015599:Trauma Severity Indices; D014947:Wounds and Injuries / Q000453:epidemiology*