This morning, I was sitting at a stop light and heard a terrifying screeching. A black sedan headed my direction from the oncoming lane was careening across six lanes of traffic, with squealing brakes and tires, through the intersection, flying forward some 75 or so yards and finally landing against a pole with a mighty impact. Terror shot through everyone in the vicinity.

Green means go. The problem was that there were cars in the way.

What happened? The woman behind the wheel had been driving 45 mph and saw a green light ahead of her. Green means go. The problem was that there were cars in the way. By the time she took note of this, it was too late. She swerved into the next lane and lost control of her car. Even an hour later, there were still police cars, fire engines, and ambulances everywhere. It was very bad, but it could have been much worse.

She should have been more alert, of course. But like most drivers, her attention was entirely on the fact that the green light gave her the right of way, regardless of what was around her. Green gives her the legal right to drive forward. It doesn't guarantee that doing so will be safe.