Well, it maybe hard for a machine to visually identify a traffic light, but that's hardly the only way. In the "Internet of Things" vision, traffic lights are one of the first things to be connected to the network for traffic shaping. Hence, autonomous networked cars will be actually aware of not only the closest traffic light, but of all traffic lights in a certain radius. Assuming John Leonard lives another 30 years, I find it hard to believe, that similar functionality won't be implemented by then.
Then again, one could argue that networked traffic lights won't span the globe, and thus autonomous cars will be bounded in certain geographical areas. That was true for a lot of modes of transportation originally though, and eventually it will be minimized.