Just to echo the OP: Was biking with my brother and a friend in San Fran. The friend crashed when his tire got stuck in the trolley track. Then my brother crash

Just to echo the OP: Was biking with my brother and a friend in San Fran. The friend crashed when his tire got stuck in the trolley track. Then my brother crashed same way while looking back to see what happened. Always wear a helmet; Especially in San Fran