On left turns, the closer you can match the speed of motor traffic, the easier it will be. If the fastest you can go is 20 and the cars are going 45, forget it: you're better off using the cross walk. But if you can go 25 and the cars are going 30, it can be done: ramp up speed, look back and find a gap, put out your left hand and slot in. Take the left like you're driving a car.