Study Details

Study Title: Operational and Safety Trade-offs: Reducing Freeway Lane and Shoulder Width to Permit an Additional Lane

Authors: Dixon et al.

Publication Date: JAN, 2016

Abstract: This research effort identified the operational and safety implications of using reduced lane and shoulder widths for a variety of urban freeway configurations. The authors used speed, crash, and geometric data for freeways in Dallas, Houston, and San Antonio, Texas. The operational analysis identified an increase of about 2.2 mph in speed for a 12-ft lane as compared to an 11-ft lane. The shoulder width is significant when the adjacent lane is 11-ft wide, but not when it is 12-ft wide which suggests that left shoulder width is more important with a reduced lane width. Operating speeds on Texas freeways are 2 mph lower during night time (with roadside lighting present) than during the day. Speeds were higher (by 1.5 mph) on the weekends (Saturday) than on the week day studied (Wednesday). The safety analysis determined a crash difference when comparing freeways with 12-ft to 11-ft lanes. There is a reduction in KAB crashes that ranges from 5% for 2-lane freeways up to 12% for 5-lane freeways. Similarly, there are crash reductions associated with each additional lane, increased left shoulder widths, and increased right shoulder widths. This paper concludes with a series of sample problem scenarios that collectively demonstrate how the resulting operating speed and crash prediction models allow practitioners to assess tradeoffs that can be expected when modifying urban freeway lane and shoulder widths.

Study Citation: Dixon, K., K. Fitzpatrick, and R. Avelar. "Operational and Safety Trade-offs: Reducing Freeway Lane and Shoulder Width to Permit an Additional Lane". Presented at the 95th Annual Meeting of the Transportation Research Board, Paper No. 16-6761, Washington, D.C., (2016).

Related Citations: Dixon, K., K. Fitzpatrick, R. Avelar, M. Perez, S. Ranft, R. Stevens, S. Venglar, and T. Voigt. "Reducing Lane and Shoulder Width to Permit an Additional Lane on a Freeway: Technical Report." Report No. FHWA/TX-15/0-6811-1, Texas Department of Transportation, Austin, TX, (March 2015).


CMFs Associated With This Study

Category: Roadway

Countermeasure: Increase lane width from 11 feet to 12 feet

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.88 12 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban
0.93 7 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban
0.9 10 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban
0.95 5 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban

Countermeasure: Install an additional lane

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.76 24 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban
0.75 25 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban
0.74 26 3 Stars All Fatal,Serious injury,Minor injury Principal Arterial Other Freeways and Expressways Urban

Category:Shoulder treatments

Countermeasure: Widen shoulder

CMF CRF(%) Quality Crash Type Crash Severity Roadway Type Area Type
0.95 5 3 Stars All All Principal Arterial Other Freeways and Expressways Urban
0.91 9 3 Stars All All Principal Arterial Other Freeways and Expressways Urban

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