Monash University Accident Research Centre - Report #21 - 1991
Author: M. Taylor
Full report in .pdf format [2.5MB]
This study examined the mobility performance of partially and fully controlled right turn phases at signalised intersections. The objective of the study was to estimate the changes in delays, congestion and intersection performance resulting from changes in right turn control type. The evaluation of the safety performance of these right turn signals is covered in a separate report (Part 1).
The mobility study had three main parts: (1) the identification of intersections where different types of controls had been installed, (2) modelling of the mobility performance of Typical' intersections under a range of traffic conditions, and (3) modelling the performance of a selection of case study intersections, drawn from the sites found in phase (1).
The major part of the study of mobility at signalised junctions was based on the analysis of typical or stereotype intersections, representing different design standards, geometrical and traffic features. Five stereotype intersections were considered: (1) a four-leg junction of a high design standard, (2) a four-leg junction of 'poor' design standard, (3) a high standard T-junction, (4) a 'poor' design standard T-junction and (5) a four-leg junction of medium design standard.
This study found that there are reductions in overall mobility when right turn controls are introduced, for most traffic conditions except some instances of peak period operations. Further, full control of right turns yields poorer intersection performance than partial control under virtually all conditions, although the differences in performance were slight, especially under off-peak conditions, and are most unlikely to negate the significant safety advantages reported in Part 1 of this study.
Under peak period conditions, right turn phases gave mobility benefits for unsaturated intersections. Introduction of turning controls leads to increased cycle times and more complex phasing arrangements, so that once saturation was reached the use of more stringent turn controls exacerbated the level of saturation. No advantage in mobility performance could then be found.
Under off-peak period conditions, the differences in mobility performance between alternative right turn control regimes are small, suggesting that the considerable safety benefits found for full control (see Part 1 of this study) should be of primary concern. At peak periods, partial control may offer mobility advantages, but at all other times the gains from partial control instead of full control are not significant.