Do drivers reduce their headway to a lead vehicle because of the presence of platoons in traffic? A conformity study conducted within a simulator

Electronically coupled platoons of vehicles have the potential to increase the efficiency of transport on unmodified motorways. In doing so, the time headway (THW) between participating vehicles will be dramatically reduced. The present study investigated whether drivers are willing to keep a THW smaller than their preferred one to conform to the norm established by the presence of platoons holding short THWs. Firstly, two constructs had to be distinguished to answer this question: the preferred THWs represent a range of THWs drivers feel safe with and the THW that is indeed adopted by drivers in a given situation (adopted THW). Secondly, comparing adopted and preferred THW informs about whether drivers would adopt a THW beyond their preferred one as a result of the influence of platoons. Forty-two participants were asked to follow a lead vehicle (LV) in three different traffic conditions. In two conditions, there was a platoon of vehicles in the inside lane, where the THW between the vehicles was either large (THW = 1.0 s) or short (THW = 0.3 s). In a third baseline drive, the LV was the only vehicle present. Preferred THW was assessed after each traffic conditions with the psychophysical method of limits. Results show a consistency of preferred THW and there is a significant difference in adopted THW values throughout the conditions, which supports the idea of two distinct constructs. Further, participants' minimum adopted THW did not drop under but was very close to the minimum preferred THW in condition THW03. It can be concluded that platoons could lead drivers to drive closer to their limits. Further studies need to investigate if in other conditions, drivers would go below their limit and in this case, consequences on the drivers (e.g. in terms of safety, workload and performance) will also require further investigations.