Influence of midblock U-turn facility on traffic flow reduction and its effect on kinematic wave propagation

Abstract

Midblock median opening facilities are constructed on multilane highways in Malaysia solely for the purpose of facilitating U-turn movements along federal road segments. In Malaysia, a persistence problem of traffic conflicts and congestion at multi-lane highway intersections forced the federal authorities to invest in midblock median opening facilities as traffic conflict reduction mechanism. Although the authorities have succeeded in reducing traffic conflicts at intersections, the realisation of maximum traffic flow at the midblock zone is in doubt. The aim of the study is to determine influence of midblock U-turn facility on traffic flow reduction and its effect on kinematic wave propagation. The objectives are to determine traffic volume, speeds as well as vehicle types at the midblock and free-flow zones for both directional flows. As well as examine the effect of traffic flow reduction on kinematic wave propagation. It was assumed that traffic density was a resultant of speed and traffic flow hence not directly affected by midblock zone. It implies that traffic flow reduction was fully the result of speed changes. Where traffic flow reduction has occurred, the ensuing kinematic wave propagation would be investigated in order to determine whether it is a rarefaction wave or traffic shockwave. Vehicle types, traffic volumes and vehicles speeds were collected using automatic traffic counters at each directional flow of the four surveyed sites for six months. The survey data were collated and analysed. Passenger car equivalent values were modified and used to convert traffic volume to flow. Results show that midblock facility would cause about 4 per cent reduction in traffic flow at the diverging section and 10 per cent reduction in traffic at the merging section. The findings give traffic capacity values under different scenarios and can be incorporated into a wider strategy for dynamic traffic management. Findings from kinematic wave propagations confirm that midblock facility would cause traffic shock wave at the merging not the diverging section. Estimated traffic flow reduction from the study can be used as evidence to highlight the need to redesign midblock facility in Malaysia. One possible solution among others is to incorporate appropriate acceleration lane at the merging section thereby minimising the potential for vehicle collision. The study concluded that midblock facility will cause traffic significant flow reduction.