Designing a licence plate for memorability

Good memorability of licence plates is important in those cases where licence plates are viewed for a brief period of time and the information is essential for police investigations. The purpose of the current study was to design a new Dutch licence plate that could be remembered well. A memory experiment was conducted, in which 16 different character arrangements were presented for both 450 and 550 ms to 48 participants with ages varying between 20 and 57 years. Participants had to rehearse the stimuli for 6 s, after which they had to be written down. Based on literature on short-term memory for serial order, character arrangements differed on three dimensions: 1) number of alternations between letters and digits; 2) letter to digit ratio; and 3) equality of group size. Results showed that number of alternations between characters of different categories affected memory performance the most. Letter to digit ratio and equality of group size affected memory performance to a lesser, but still significant, extent. A significant interaction between the latter two factors indicates that equal groups only lead to fewer memory errors when more than three letters are used. With three or fewer letters, group size is not a significant factor any more. Based on these results, a new licence plate for Dutch vehicles was recommended, which was subsequently adopted.