INTEGRATING OF ARTERIAL SIGNAL AND FREEWAY OFF-RAMP CONTROLS FOR COMMUTING CORRIDORS

Abstract

Congestion at the downstream intersections of a main arterial often causes the formation of traffic queue from the off-ramp back to the freeway mainline, and the resulting queue spillback will then substantially reduce the freeway capacity. To contend with such a critical issue, this study proposes an integrated operating system which consists of O-D demand estimation functions, pre-timed signal optimization models, and real-time signal control strategies.

The primary function of O-D estimation is to identify the traffic demand pattern at the freeway off-ramp and its connected arterial segment. Since the congestion patterns are usually caused by the green time allocation between off-ramp turning flows and local through traffic, conventional algorithms for signal design geared to best facilitate the arterial flows are likely to fall short of providing sufficient green duration to heavy off-ramp flows. Hence, to prevent queue spillover at freeway off-ramps, the system first detects the primary traveling paths of both off-ramp and local arterial flows so as to maximize the efficiency of the signal progression system and to best utilize the capacity of the intersections within the impact region of a freeway interchange.

Based on the identified demand patterns, the second part of the system constructs two sequential models to optimize the pre-timed signal plan for the target intersections. With the objective of maximizing intersection capacity, the first model is developed to optimize the green splits and cycle length at each impacted intersection. Then, the second model optimizes the signal offset and phase sequence for each intersection to satisfy the progression needs of the identified critical path-flows.

To contend with the fluctuation of traffic flows, this study further develops a real-time control strategy which consists of three core modules, including off-ramp queue estimation, arterial adaptive control, and off-ramp priority control. If no freeway breakdown caused by off-ramp queue spillover is predicted, the arterial adaptive control module will be implemented to dynamically adjust the intersection signal timings and offsets. Otherwise the system will activate its off-ramp priority control module to offer green extensions and progression priority to the off-ramp flows.