Effect of Road Narrowing on Junction Capacity Using Microsimulation

Traffic-calming measures have been widely used in urban areas, aiming sometimes to reduce vehicle flow rates but primarily to reduce speed and, hence, the number and severity of traffic accidents. The one-way priority working road narrowing (throttle) is a widely used traffic-calming technique. However, installation of such a measure close to a junction can severely affect the operation and level of service, especially for the major road movements. There is a lack of research, guidance, and design standards to inform the effective deployment of such a measure. This paper starts by providing a summary of the terms and standards used in practice for traffic-calming measures, primarily focusing on the use of throttles. The paper aims to investigate the relationship between throttle spacing ($X$) from a priority junction and the “major-to-major movement” driver’s delay for different flow levels using the S-Paramics microsimulation model. The model was calibrated and validated with field data collected from a selected site within Greater Manchester for two separate days by using camcorders. Cooperative behavior between drivers (i.e., those with priority who gave way to others) was noticed and analyzed accordingly. Different scenarios were used to test the effects on delays and queues for various parameters including throttle spacing ($X$), major-arms flow level, and reversing the direction of priority. The findings show that there is a direct relationship between throttle spacing ($X$) and the level of delay for traffic on the major arms of the junction. It is also shown that for a minor arm with two-way flow level of up to 500 vehicles per hour ($\mathrm{veh}/\mathrm{h}$), the throttle spacing should be at least 30 m. Once the flow level reaches $700\mathrm{veh}/\mathrm{h}$, the spacing should be at least 50 m, whereas for flow levels of $800\mathrm{veh}/\mathrm{h}$, the spacing should be at least 100 meters. Alternatively, consideration should be given to reversing the priority given to traffic by using MJTP (major-arm traffic has priority) operation rather than MNTP (minor-arm traffic has priority) when less throttle spacing ($X$) is required, to minimize delays to traffic on the major arms.