Evaluation of different types of dynamic speed display signs

There is considerable interest in new technology that has the potential for reducing speed and thus improving traffic safety. This paper investigates the effectiveness of three different types of dynamic speed display sign (DSDS) for reducing speed: (i) a standard DSDS with numeric values corresponding to the driver's actual speed; (ii) a standard DSDS with numeric values corresponding to the driver's actual speed which are highlighted in red or green depending on whether the car driver complied with or exceeded the local speed limit; and (iii) a verbal coloured DSDS where the word THANK YOU appears in green letters when the car driver kept the speed limit and the word SLOW in red letters when the driver exceeded the speed limit. The different signs were installed one after the other at the same test site in a residential area in Berlin (Germany). Speed was measured with a radar device continuously 1 month before the installation, 2-3 months during operation, and 1-6 months after dismounting. All DSDS reduce average speed, 85th percentage speed, and the percentage of vehicles exceeding the speed limit. After dismounting speed levels returned to its baseline for all three DSDS. The verbal coloured DSDS reduces speed most followed by the numeric coloured DSDS and the numeric DSDS. The verbal coloured DSDS also exhibits no habituation effects in contrast to both numeric DSDS. Speed distribution analysis revealed that the verbal coloured DSDS influence speed from the first speed interval above the posted speed limit whereas both numeric DSDS influence speed only from the second speed interval onwards. The differences between the DSDS are explained with different priming mechanism initiated by traffic signs.