Calgarians got a refresher on winter driving Tuesday night. The commute on city roads got trickier by the hour. The struggle became very real for motorists on Shaganappi Trail by Dalhousie Drive N.W. A section north of here had to be shut down because vehicles and city transit buses simply couldn't get up the hill. Calgary police say that for a time, a crash was being reported about every four minutes, including a vehicle that slid down the street and a hill right into a community centre in the northwest. Between 4 and 9 p.m. on Tuesday, there were 71 crashes reported, 10 of them involving minor injuries. Calgary, as well as much of southern and central Alberta, remains under a snowfall warning.