Supporting driver headway choice: The effects of discrete headway feedback when following headway instructions Malte Risto a, * , Marieke H. Martens a, b a University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands b TNO, P.O. Box 23, 3769 ZG Soesterberg, The Netherlands article info Article history: Received 22 October 2013 Accepted 10 February 2014 Keywords: Headway instructions Headway feedback Headway choice accuracy abstract With specific headway instructions drivers are not able to attain the exact headways as instructed. In this study, the effects of discrete headway feedback (and the direction of headway adjustment) on headway accuracy for drivers carrying out time headway instructions were assessed experimentally. Two groups of each 10 participants (one receiving headway feedback; one control) carried out headway instructions in a driving simulator; increasing and decreasing their headway to a target headway of 2 s at speeds of 50, 80, and 100 km/h. The difference between the instructed and chosen headway was a measure for headway accuracy. The feedback group heard a sound signal at the moment that they crossed the dis- tance of the instructed headway. Unsupported participants showed no significant difference in headway accuracy when increasing or decreasing headways. Discrete headway feedback had varying effects on headway choice accuracy. When participants decreased their headway, feedback led to higher accuracy. When increasing their headway, feedback led to a lower accuracy, compared to no headway feedback. Support did not affect driver’s performance in maintaining the chosen headway. The present results suggest that (a) in its current form discrete headway feedback is not sufficient to improve the overall accuracy of chosen headways when carrying out headway instructions; (b) the effect of discrete headway feedback depends on the direction of headway adjustment. Ó 2014 Elsevier Ltd and The Ergonomics Society. All rights reserved. 1. Introduction In traffic, keeping the right inter-vehicle distance (or headway) is essential for the safety of drivers as well as the overall efficiency of the traffic system. In combination with driver inattention, short following headways have been identified as a main contributing factor in the occurrence of rear-end collisions (Campbell et al., 2003; Knipling, 1993). The literature on driver support systems has acknowledged the need to reduce the frequency of short headway driving in order to increase driver safety (Ben-Yaacov et al., 2002; Shinar and Schechtman, 2002). Besides its conse- quences for traffic safety, short headways also reduce the ability to recover from disturbances in traffic flow (e.g. sudden braking ma- noeuvres) and can facilitate the propagation of shockwaves (Sugiyama et al., 2008). Furthermore, with an expected increase in road demand the efficient use of the existing transport infrastruc- ture will only gain in importance. From the perspective of traffic efficiency, large headways have undesirable consequences as well. Average time headway of all vehicles on a road is directly related to the maximum road capacity in that larger headways reduce the capacity of the road and thereby decrease throughput (Kesting et al., 2009; Marsden et al., 2001). In some situations asking drivers to keep shorter time headways can benefit throughput. However, the approach bears the risk that drivers may attain unsafe headways due to errors in the estimation of an advised headway. To promote safe driving, licensing manuals usually recommend time headways of 2 s or more. Drivers ought to use these headway values as a reference for their headway choice. To be able to do so, drivers themselves have to estimate their headway in seconds, in order to compare their estimates to the recommended time headway. However, studies have shown that drivers have diffi- culties in accurately estimating time headways. In a real road experiment by Ben-Yaacov et al. (2002), drivers were instructed to maintain a time headway of 1 s. Results showed that drivers kept significantly shorter headways than instructed. Risto and Martens (2013) instructed drivers in a driving simulator and on a real road to attain headways of 1,1.5 or 2 s at varying speeds. On average the headways chosen by participants following the instructions were smaller than instructed in both environments. In another real road * Corresponding author. Tel.: þ31 (0)53 489 4004; fax: þ31 (0)53 489 4040. E-mail addresses: m.risto@utwente.nl (M. Risto), marieke.martens@tno.nl (M. H. Martens). Contents lists available at ScienceDirect Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo http://dx.doi.org/10.1016/j.apergo.2014.02.003 0003-6870/Ó 2014 Elsevier Ltd and The Ergonomics Society. All rights reserved. Applied Ergonomics 45 (2014) 1167e1173