Headway minimum

Tdmros, J., Nilsson, L., Ostlund, J., and Kircher, A. 2002. Effects of ACC on driver behaviour, workload and acceptance in relation to minimum time headway. Proceedings from the 9th World Congress on Intelligent Transport Systems, Chicago. 

Taieb-Maimon, M. and D. Shinar (2001). Minimum and comfortable driving headways reality versus perception. Hum. Fact, 43(1), 159-172. 

Drivers estimation of minimum safe headways and comfortable headways... 

Figure 5-5. Mean, 1 standard deviation, and 1.98 standard deviations of distance headways (left panel) and time headways (right panel) kept by drivers who were asked to maintain a minimum safe headway (derived from data from Taieb-Maimon and Shinar, 2001).

The results were disappointing. The correlation between the two measures was essentially zero. Not only that, but for 7 of the 30 people who participated in the study the average perception reaction time under optimal conditions was actually longer than the minimum headway they kept on the road. For these drivers, were that car ahead to stop suddenly, the likelihood of colliding with it was very high. However, this lack of correlation between the headways drivers keep and their brake reaction time should be investigated further, since at least one study found a positive correlation between the two, showing that those who keep short headways have quicker reaction times (Van Winsum and Brouwer, 1997). 

One method to improve headway judgments would be to have in-vehicle headway-o-meters (just as we have speed-o-meters). The belief that minimum headways can be regulated and enforced rests on the assumption that drivers are capable of either directly perceiving or correctly estimating their headways. We do not make that assumption with respect to speed and that is why we have speedometers in our cars. The research on drivers headway judgments shows that we are incapable of this task too, and need some kind of an aid. 

Wasted track capacity. One could take a different viewpoint and try to assess how efficiently the existing signalling makes use of the available track. Assuming a certain traffic pattern, the plug-in measures the minimum amount of free track observed during a traffic scenario. The fundamental idea is that line interference and inefficient signalling tend to increase train headways. 

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