  Freeway on-ramps are typical bottlenecks in the freeway network due to the
frequent disturbances caused by their associated merging, weaving, and
lane-changing behaviors. With real-time communication and precise motion
control, Connected and Autonomous Vehicles (CAVs) provide an opportunity to
substantially enhance the traffic operational performance of on-ramp
bottlenecks. In this paper, we propose an upper-level control strategy to
coordinate the two traffic streams at on-ramp merging through proactive gap
creation and platoon formation. The coordination consists of three components:
(1) mainline vehicles proactively decelerate to create large merging gaps; (2)
ramp vehicles form platoons before entering the main road; (3) the gaps created
on the main road and the platoons formed on the ramp are coordinated with each
other in terms of size, speed, and arrival time. The coordination is formulated
as a constrained optimization problem, incorporating both macroscopic and
microscopic traffic flow models, for flow-level efficiency gains. The model
uses traffic state parameters as inputs and determines the optimal coordination
plan adaptive to real-time traffic conditions. The benefits of the proposed
coordination are demonstrated through an illustrative case study. Results show
that the coordination is compatible with real-world implementation and can
substantially improve the overall efficiency of on-ramp merging, especially
under high traffic volume conditions, where recurrent traffic congestion is
prevented, and merging throughput increased.
