Betsey Johnson witnessed an emergency last week — and not the fashion kind. The designer, 70, was dressing mannequins with granddaughter Layla in Macy’s windows, to launch a new line of dresses, when a car racing along West 34th Street dramatically crashed into an ambulance. “The car totally lost control and went careening into the ambulance,” a Johnson pal told us. “Then the driver tried to get away, but his wheel fell off and he couldn’t escape.” Police sources confirmed the accident. There were no arrests or injuries.