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Research Reports

MRR 198: Content Evaluation Of Authorised Left Turn (ALT)

Authorised Left Turn (ALT) traffic control had been gazette in Road Transport Act in 2012. Nevertheless, the efficiency of this ALT has not been evaluated. This study was undertaken to assess the ALT from safety and efficiency perspective. Three junctions with a total of 8 approaches were installed with ALT traffic control. On-site data such as volume, conflict and turning movements were collected for before and after installation periods in order to compare the efficiency of ALT.  
 
The results on the three junctions show that ALT had improved the capacity at the certain degree. Nonetheless, most of the left turn on red movements violated the ALT rule of which motorists need to come to a complete stop before performing ALT. No prominent or serious conflict was detected during the study periods. This may be due to the short time of implementation where the novelty factor might influence the behaviour of motorists at the junction. It is also to highlight here that no analysis on pedestrian and ALT vehicle due to an absence of data.  

Several suggestions were recommended to add value to the current technical guideline of ALT such as the importance of ALT standard sign provision, no protected left turn green arrow as well as a minimum of two receiving lanes.  
 
By and large, the installation of ALT traffic control did show some improvement in the traffic capacity, but there was also evidence of unsafe interactions collected on site. It is hoped that with the highlighting of the problems and implementing of the right countermeasures, the ALT traffic control can benefit the public and road users.

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MRR198Content Evaluation of Authorised Left Turn (ALT).pdf
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