Car Platooning String Stability Analysis for Automatic Longitudinal Control in Automated Highway Systems

Forming car platoons and managing them through automation allows cars to travel with shorter gaps, which contributes to increasing road throughput, lowering fuel use, and cutting emissions. However, minimizing inter-vehicle spacing must comply with the principle of string stability. This paper introduces a Linear Quadratic Integral Regulator (LQIR) tailored for the Adaptive Cruise Control strategy within a platoon of cars. The regulator integrates two additional states to guarantee both effective tracking and preservation of string stability. Furthermore, a heuristic procedure is proposed to select the Constant Time Headway Policy (CTHP) as a means of achieving stability. The selection routine combines an examination of the magnitude frequency response with an inspection of the pole–zero distribution of the spacing-error transfer function between successive cars. Using this approach, a comparative investigation is carried out for two autonomous Adaptive Cruise Control architectures: one based on absolute position feedback and another on measured inter-vehicle distance.