Display helps drivers keep their distance

Maintaining distance from the car in front of you improves safety, but it also reduces road capacity. An experimental new display promises higher traffic density as well as fewer collisions.

(Photo: Wikimedia - Harrikkamies)
(Photo: Wikimedia - Harrikkamies)

Over a million people die in traffic accidents each year, according to the World Health Organisation. And most crashes are rear-end collisions. Either drivers start breaking too late or they're following the car in front too closely.

PhD research by Dr. Mehdi Saffarian at the Faculty of Mechanical, Maritime and Materials Engineering has shown that even a distraction as short as half of a second can dramatically increase the risk of a crash. Also, he found that brake lights helped to reduce reaction times when the car in front braked moderately (1.7 m/s2). In the case of an emergency stop (6.5 m/s2) the warning effect of the brake lights contributed very little to the reaction time. Saffarian completed his tests in a driving simulator with 22 students with two or more years of driving experience.

The RNWD signalling emergency. (Photo: PhD thesis)
The RNWD signalling emergency. (Photo: PhD thesis)


A newly developed Rear Window Notification Display (RWND) was added to the brake lights. It shows with a horizontal bar whether the lead car brakes (left, in red) or accelerates (right, in green). The vertical arrows indicate if it is safe to shorten the distance (up) or fall back (down). The RNWD could be an addition to the brake lights of the car in front as well as a projected display on the window screen of the trailing car.

The ideal distance or time headway (THW) is defined as the distance between vehicles divided by the speed of the trailing car. It is considered a key indicator of the road capacity and correlates with a driver's perception of risk. The ideal THW lies between 1.0 and 1.5 seconds.

Driver awareness

Saffarian compared the RNWD and brake lights as follows. 'Both result in driver awareness and action. The distinguishing factor is that, unlike traditional brake lights, the RNWD communicates the magnitude of acceleration and deceleration to the driver, helping the drive to better implement the start, duration and level of control action in response to lead-car behaviour.'

In his experimentation, after a five minute simulation drive to get used to the system and the new display, the drivers had to follow a lead car (with RWND) for about 10 minutes during which the speed varied from 15 to 110 km/hour. The display instructed them to speed up or slowdown in order to maintain the ideal distance from the car in front.

The ensuing questionnaire showed that the student drivers had little difficulty interpreting the display and did not regard it as a distraction. Most of them agreed that the bars and arrows helped them to keep the right distance to the car in front.

The RNWD has a remarkable effect on following distance. Nearly all drivers follow the lead car with one to two second distance between them (as measured in THW). Without the display, the spread is much larger (one to four seconds), and thus the gap between cars is bigger.

Driver's choice

The advantage, higher traffic density, does come at a price. Keeping the right distance goes hand in hand with more acceleration and braking actions, resulting in a busier driver. The fuel consumption is likely higher when compared to the constant speed of a car on cruise control.

Whether or when to use the display, remains the driver's choice. Saffarian reminds us that drivers like technologies that enhance their perception and control. But they're not fond on technologies taking over control

Mehdi Saffarian, 'Understanding and improving driving performance by removing and adding visual information', PhD supervisors Professor Frans van der Helm, Dr. Joost de Winter and Dr. Riender Happee, Defence on March 10, 2017.