Effectiveness of Experimental Left-Turn Sign Usage in Terms of Crashes and Analyzing Severity of Left Turn Crashes in Alaska

Osama A. Abaza, Tanay Datta Chowdhury*
Civil Engineering Department, University of Alaska Anchorage, 2900 Spirit Drive, Anchorage, Alaska 99508, USA

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© 2018 Abaza and Chowdhury.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at Civil Engineering Department, University of Alaska Anchorage, 2900 Spirit Drive, Anchorage, Alaska, USA, Tel: (907)-346-0055; E-mail:



Ensuring safety in the roadways is one of the critical tasks as crashes generally bourgeoning rapidly all over the world. Because the turning movements of vehicles are important to motorist safety, Alaska Department of Transportation and Public Facilities (DOT & PF) experimented with left-turn traffic signs at six signalized intersections in the city of Anchorage, Alaska.


The study aims at evaluating effectiveness of the left turn signs and analyze severity of left turning crashes.


Preliminary statistical analysis and Binomial Logistic regression are applied for analyzing the data.


Preliminary analysis of before (1994-1998) and after (2005-2012) crash frequency data showed a reduction of crashes at four of the six intersections studied per 1000 entering vehicles. An analysis of crashes per year indicated that left-turn crashes at five intersections declined significantly after 2009, showing an improvement in safety. Pavement markings with signage were vitally important in contributing to improve compliance rates. Covering the pavement markings with snow and ice in Alaska for long periods signify the importance of installation of left turn signage to reduce crashes at studied intersections. Logistic regression analysis revealed that improper turn and lane usage, careless driving and angle type crashes were the most significant factors contributing to PDO crashes. Dry pavement conditions as well as almost all day except evening peak and off peak period influence PDO crashes in Alaska.


Left-turn signs were effective in reducing crashes, and proper maintenance of road markings as well as driver awareness increased the effectiveness of signage and level of safety for left turning vehicles.

Keywords: Left-turn traffic signs, Crash frequency, Compliance rate, Crash severity, Logistic regression analysis, Alaska.