The aim of this study was to find out if the colour contrast of a vest worn by cyclists improved their visibility for motorists in a simulated car driving task. Background. In several cases involving collisions between cars and cyclists, car drivers failed to detect the latter in time to avoid collision because of their low visibility or sensory conspicuity. We studied the effect of a yellow cyclist jacket on the detection of cyclists by motorists in an urban environment during daylight hours. Method. 43 motorists performed a vulnerable road user detection task in a car-driving simulator. They had to detect cyclists and pedestrians in an urban setting, to locate them and estimate how difficult it was to detect them. Cyclists' levels of visibility for car drivers were either high or low in critical situations. The cyclists were dressed in grey and wore yellow jackets, or they did not. Results Motorists detected cyclists wearing jackets at a greater distance, but only in those situations previously identified as being of high cyclist visibility. Several mechanisms underlying the sensory conspicuity of cyclists for car drivers were considered. Motorists were able to estimate cyclist visibility correctly based on the type of situation, but were unable to evaluate the jacket's effect on cyclist visibility. Conclusion The attentional selection of a cyclist in the road environment during car driving depends partly on bottom-up processing (such as saliency related to colour contrast). However, the yellow cyclist jacket proved to be insufficient as a visibility aid.