Wait for a safe gap in oncoming traffic, then make the left turn. Five areas (city, desert, snow, coast and downtown at night) each add a lane to cross on every level, from one up to ten, while denser traffic, speeders in the fast lane, pedestrians in your crosswalk, motorcycles splitting lanes or riding the wrong way, trucks and drivers merging into your road make every crossing harder.
Oncoming vehicles per minute at each level, with each area marked along the top. The highlighted bar is the level you're on; click a bar on the start screen to pick that level.