Speed vs. acceleration advice for advisory cooperative driving

Cooperation among vehicles is aimed to create a smooth traffic flow and minimize shockwaves and traffic jams. Cooperative adaptive cruise control (C-ACC) systems calculate acceleration values and exchange them between vehicles to maintain appropriate speed and headway. Before C-ACC technology gets mature, cooperative driving may already be made possible by advisory systems, keeping the drivers in the loop. While C-ACC systems are based on acceleration values, in conventional vehicles one of the main sources of information to the driver for maintaining appropriate speed is the speedometer. In this paper we present a study addressing the question of whether advisory systems should employ acceleration or speed values to advise the driver. Subjective results showed that preferences were approximately equally split between both systems. Objective results showed that acceleration advice caused more uniform speed in heavy traffic and more stable distance keeping, that speed advice led to more efficient accelerator pedal changes, and that letting drivers use their preferred advice resulted in a shorter time headway leading to a more effective traffic flow.