The overrepresentation of very old people (75 or older) in pedestrian crash statistics raises the issue of the effects of normal and pathologic ageing on gap-selection difficulties during street crossing. The present study focused on Alzheimer disease, a condition commonly associated with cognitive declines detrimental to daily life activities such as crossing the street. Twenty-five participants with mild dementia and 33 controls carried out a street-crossing task in a simulated environment. They also took a battery of cognitive tests. The mild-dementia group was more likely than the control group to make decisions that led to collisions with approaching cars, especially when the traffic was coming from 2 directions and they were in the far lane. Regression analyses demonstrated that the increased likelihood of collisions in the dementia group was associated with impairments in processing-speed and visual-attention abilities assessed on the Useful Field of View test. This test has already proven useful for predicting driving outcomes, falls, and street-crossing difficulties in healthy old adults, and among drivers with Alzheimer disease. Clinicians are encouraged to use it to help estimate whether a patient can drive, walk, and cross a street safely.