Adaptive Cruise Control System Design and Its Impact on Traffic Flow

This study resolves the controversy over the stability of constant time-gap policy for highway traffic flow. Previous studies left doubt as to the effectiveness of constant time-gap policies and whether they maintain stability in all traffic conditions. The results of this study prove that the constant time-gap policy is in fact stable to a limit. At this limit, depending on the boundary conditions, conditions lose their stability. This study develops alternative ways to maintain the balance between safety and traffic flow for ACC vehicles that does not rely on constant time-gap policies. New spacing policies will create more stability, and therefore safer conditions, and allow for greater traffic capacity. A summary of major symbols used in the report i x inertial longitudinal position of i th vehicle i l length of i th vehicle i ε spacing error for i th vehicle i δ spacing error for i th vehicle des L desired inter-vehicle spacing at zero speed τ time constant for 1 st order lag model of acceleration tracking by vehicle h time-gap λ control gain used in CTG, VTG and other control laws Q traffic flow volume rate ρ traffic density in vehicles/ meter f v speed parameter used in VTG spacing policy m ρ density parameter used in VTG spacing policy) (i x g & desired spacing as a function of vehicle velocity The desired spacing that an ACC vehicle attempts to maintain with respect to the preceding vehicle is called the spacing policy. In Fig. 1, the desired spacing is the desired value of 1 1 − − − − i i i x x l. The desired distance is typically a function of the ACC vehicle velocity [1] but could also be a constant or a function of other variables such as the relative velocity. i x 1 − i x 1 + i x 1 − i l Fig. 1 String of adaptive cruise control vehicles The spacing policy is important because it determines vehicle safety, traffic flow as well as user-acceptance of the ACC system. The most common spacing policy used in ACC systems by researchers as well as automotive manufacturers is the constant time-gap spacing policy. The constant time-gap spacing policy is given by L x h i i i + + = & ε δ (1) where the inter-vehicle spacing is 1 − − = i i i x x ε …