BACKGROUND: Pedestrians and cyclists account for nearly one in three of all road users killed and seriously injured in road traffic crashes. Late detection of other road users is one of the basic driver failures responsible for collisions. Aids to improve pedestrians and cyclist visibility have been used to avert potential collisions. However, the impact of these strategies on drivers' responses, and on pedestrian and cyclist safety is not known. OBJECTIVES: 1. To quantify the effect of visibility aids vs no visibility aids, and of different visibility aids on the occurrence of pedestrian and cyclist-motor vehicle collisions and injuries. 2. To quantify the effect of visibility aids vs no visibility aids, and of different visibility aids on drivers' detection and recognition responses. SEARCH STRATEGY: We searched the Cochrane Controlled Trials Register, the Injuries Group Specialised Register, MEDLINE, TRANSPORT, TRANSDOC from ECMT (European Conference of Ministers of Transport), IRRD (International Road Research Documentation), TRIS (Transportation Research Information Services), NRR (National Research Register), PsycInfo and PsycLit. SELECTION CRITERIA: 1. Randomised controlled trials and controlled before/after trials of the effect of visibility aids on the occurrence of pedestrian and cyclist-motor collisions and injuries. 2. Randomised controlled trials of the effect of visibility aids on drivers' detection and recognition responses. This included trials where the order of presentation of visibility aids was randomised or balanced using a Latin square design. DATA COLLECTION AND ANALYSIS: Two reviewers independently screened records, extracted data and assessed trial quality. MAIN RESULTS: We found no trials assessing the effect of visibility aids on the occurrence of pedestrian and cyclist-motor vehicle collisions and injuries. We identified 37 trials assessing the effect of visibility aids on drivers' responses. Fluorescent materials in yellow, red and orange colours improve detection and recognition in the daytime. For night-time visibility, lamps, flashing lights and retroreflective materials in red and yellow colours increase detection and recognition. Retroreflective materials arranged in a 'biomotion' configuration also enhance recognition. Substantial heterogeneity between and within the trials limited the possibility for meta-analysis. Summary statistics and descriptive summaries of the outcomes were presented for individual trials when appropriate. REVIEWER'S CONCLUSIONS: Visibility aids have the potential to increase visibility and enable drivers to detect pedestrians and cyclists earlier. Public acceptability of these strategies would merit further development. However, the effect of visibility aids on pedestrian and cyclist safety remains unknown. Studies which collect data on simple, meaningful outcomes are required.