Safety Evaluation of Geometric Design Criteria for Spacing of Entrance–Exit Ramp Sequence and Use of Auxiliary Lanes

Geometric design decisions regarding interchange and ramp spacing on freeways have traditionally taken a nominal approach to safety. This approach oversimplifies driver behavior and complex interactions between roadway geometrics, traffic operations, and safety and tends to promote a one-size-fits-all approach to evaluating design alternatives. The objective of this paper was to quantify the relationship between ramp spacing and freeway safety. Data for this study included freeway geometric features, traffic characteristics, and crash counts for 404 freeway segments in California and Washington State. A negative binomial regression modeling approach was used to explore the relationship between ramp spacing and safety. Results indicated that expected crash frequency increased as ramp spacing decreased. The expected proportion of crashes resulting in a fatality or injury appeared to decrease as ramp spacing decreased. The presence of an auxiliary lane was associated with a lower expected frequency of crashes for any given ramp spacing; the safety benefits of providing an auxiliary lane diminished as ramp spacing increased. The model results related to ramp spacing and the presence of an auxiliary lane were transformed into crash modification factors and validated through comparisons with an independent research study on weaving areas in Texas. The safety findings were generally consistent with current Green Book design criteria related to ramp spacing and the presence of an auxiliary lane. The ability to quantify the expected impact on safety of ramp spacing associated with new or modified interchanges will assist transportation agencies in making well-informed assessments of the overall benefits, impacts, and costs of freeway access decisions.

• Record URL:
  https://doi.org/10.3141/2309-02
• Summary URL:
  http://scholar.google.com/scholar_lookup?title=Safety+Evaluation+of+Geometric+Design+Criteria+for+Spacing+of+Entrance%e2%ac%1cExit+Ramp+Sequence+and+Use+of+Auxiliary+Lanes&author=T.+Le&author=R.+Porter&publication_year=2012
• Availability:
  • Find a library where document is available. Order URL: www.trb.org/Main/Blurbs/168381.aspx
• Authors:
  • Le, Thanh Q
  • Porter, Richard J
• Publication Date: 2012

Language

• English

Media Info

• Media Type: Print
• Features: Figures; References; Tables;
• Pagination: pp 12–20
• Monograph Title: Highway Design 2012
• Serial:
  • Transportation Research Record: Journal of the Transportation Research Board
  • Issue Number: 2309
  • Publisher: Transportation Research Board
  • ISSN: 0361-1981

Subject/Index Terms

• TRT Terms: Auxiliary lanes; Crash modification factors; Geometric design; Highway safety; Off ramps; On ramps; Regression analysis; Spacing
• Uncontrolled Terms: Negative binomial regression
• Geographic Terms: California; Texas; Washington (State)
• Subject Areas: Design; Highways; Safety and Human Factors; I20: Design and Planning of Transport Infrastructure; I82: Accidents and Transport Infrastructure;

Filing Info

• Accession Number: 01366024
• Record Type: Publication
• ISBN: 9780309263054
• Report/Paper Numbers: 12-2153
• Files: TRIS, TRB, ATRI
• Created Date: Mar 23 2012 8:45AM