Decision Sight Distance for Freeway Exit Ramps - a Road Safety Perspective

Design guidelines/standards and subsequent project design criteria generally stipulate that Decision Sight Distance (DSD) be provided at freeway exit terminals. In most such guidelines the DSD is to be measured to a low point on the highway at the bullnose. However, achieving DSD for exit ramps is not always realistic. Vertical and horizontal curves, the presence of retaining walls, cut slopes and other structures and fixtures present challenges that may require major design modifications (and therefore costs). This paper examines the theoretical foundation used to derive the DSD, and reflects, from a safety perspective, on whether the DSD, as defined in current design standards is appropriate and considers the implications of deviating from those standards. The paper covers a literature review of relevant design guidelines and reviews human factor parameters used to derive the DSD and positive guidance principles. The paper also examines the concept of nominal and substantive safety, which is fundamental to the topic of “departure from standards”. On that basis, the paper concludes whether the DSD to an exit ramp, as currently defined, is truly relevant in all cases to driver’s decision-­-making process. Improvements and modifications to the current DSD requirements are recommended.

• Record URL:
  http://www.tac-atc.ca/sites/default/files/conf_papers/ho.pdf
• 
• Corporate Authors:

  Transportation Association of Canada (TAC)

  401-1111 Prince of Wales Drive
  Ottawa, Ontario  Canada 
• Authors:
  • Ho, G
  • Rozental, J
  • Majstorovic, S
• Conference:
  • TAC 2016: Efficient Transportation - Managing the Demand - 2016 Conference and Exhibition of the Transportation Association of Canada
• Publication Date: 2016

Language

• English

Media Info

• Media Type: Web
• Pagination: 1 PDF file, 1.8 MB, 18p.
• Monograph Title: TAC 2016: Efficient Transportation - Managing the Demand - 2016 Conference and Exhibition of the Transportation Association of Canada

Subject/Index Terms

• TRT Terms: Conferences; Freeways; Intersection elements; Layout; Safety; Specifications; State of the art reports; Visibility distance
• Geographic Terms: Canada
• ITRD Terms: 8018: Canada; COMPLIANCE (SPECIF); 8525: Conference; 425: Exit; 2885: Layout; 2752: Motorway; 1665: Safety; 177: Specifications; 8588: State of the art report; 6793: Visibility distance
• Subject Areas: Highways; Safety and Human Factors; I21: Planning of Transport Infrastructure; I82: Accidents and Transport Infrastructure;

Filing Info

• Accession Number: 01616399
• Record Type: Publication
• Source Agency: Transportation Association of Canada (TAC)
• Files: ITRD, TAC
• Created Date: Nov 15 2016 4:56PM