The danger of riding on the left (in U.S.) side is a lot more than the change in closing speed, although that is a significant issue when rounding a blind bend with little/no shoulder and meeting up with a car.

There is also the problem that cars entering the roadway from driveways/cross streets are looking at the traffic to their left (in the U.S.), and rarely check to their right before pulling into traffic. That makes every driveway, cross street, parking lot, crosswalk, etc. a separate accident waiting to happen.

Also, intersections are a problem, with left turning traffic often not seeing the oncoming cyclist coming up on their left. Since the cyclist is on the opposite side of the road, a left turning motorist will often miss seeing the cyclist.

Also, all of the traffic control signs/devices (stop signs, road hazard signs, traffic lights, etc.) are turned away from the cyclist and they're on the opposite side of the road, making it difficult/impossible for the cyclist to see the signs/lights/etc.

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