title: Driver speed selection on high-speed two-lane highways: Comparing speed profiles between uniform and differential speed limits.
abstract: Although a considerable amount of prior research has investigated the impacts of speed limits on traffic safety and operations, much of this research, and nearly all of the research related to differential speed limits, has been specific to limited access freeways. The unique safety and operational issues on highways without access control create difficulty relating the conclusions from prior freeway-related speed limit research to 2-lane highways, particularly research on differential limits due to passing limitations and subsequent queuing. Therefore, the objective of this study was to assess differences in driver speed selection with respect to the posted speed limit on rural 2-lane highways, with a particular emphasis on the differences between uniform and differential speed limits.
  
  
  
  Data were collected from nearly 59,000 vehicles across 320 sites in Montana and 4 neighboring states. Differences in mean speeds, 85th percentile speeds, and the standard deviation in speeds for free-flowing vehicles were examined across these sites using ordinary least squares regression models.
  
  
  
  Ultimately, the results of the analysis show that the mean speed, 85th percentile speed, and variability in travel speeds for free-flowing vehicles on 2-lane highways are generally lower at locations with uniform 65 mph speed limits, compared to locations with differential limits of 70 mph for cars and 60 mph for trucks.
  
  
  
  In addition to posted speed limits, several site characteristics were shown to influence speed selection including shoulder widths, frequency of horizontal curves, percentage of the segment that included no passing zones, and hourly volumes. Differences in vehicle speed characteristics were also observed between states, indicating that speed selection may also be influenced by local factors, such as driver population or enforcement.
authors.affiliation: a Department of Civil Engineering, Construction Management, and Environmental Engineering , Northern Arizona University , Flagstaff , Arizona.
authors.forename: Brendan J
authors.initials: BJ
authors.lastname: Russo
authors.affiliation: b Department of Civil, Construction and Environmental Engineering , Iowa State University , Ames , Iowa.
authors.forename: Peter T
authors.initials: PT
authors.lastname: Savolainen
authors.affiliation: c Department of Civil and Environmental Engineering , Michigan State University , East Lansing , Michigan.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Gates
authors.affiliation: d Department of Civil and Environmental Engineering , Wayne State University , Detroit , Michigan.
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Kay
authors.affiliation: d Department of Civil and Environmental Engineering , Wayne State University , Detroit , Michigan.
authors.forename: Sterling
authors.initials: S
authors.lastname: Frazier
mesh_terms: D000054:Acceleration; D000063:Accidents, Traffic / Q000706:statistics & numerical data; D001334:Automobile Driving / Q000331:legislation & jurisprudence* / Q000523:psychology*; D004779:Environment Design / Q000706:statistics & numerical data*; D006801:Humans; D012306:Risk; D014481:United States
keywords: Speed limits; differential speed limit; free flow speed; two-lane highway; uniform speed limit