Not only in Boston or California. Here in Barcelona, too. As Aa737 says, it has to do with the power needed. As Sccutler says, it has to do with both long term cost and less failure probabilities. Actually, LEDs have a very very very long useful life.
But there's also some other good points. For example, in very sunny places, like California in this case, or in Southern Europe, when sun is low, you can find yourself in front of a traffic light of the "bulb-and-lens" system, and be absolutely unable to guess which of the three lights is on. Some twenty years ago the municipality of Barcelona tried to solve this problem installing coloured bulbs behind transparent or smerilized glasses. But LEDs are being successfully used during the last two-three years. This way, the two "lights" off look grey, and the one on, brightly coloured.
As for the cars, you have only to look for one (there's quite a lot of them) that have the two "normal" brake lights with bulb, and the third one in the middle with LEDs. If you pay attention, you'll see the center one be fully lightened about 2/10ths of a second faster. This, if your car has one of these LED lights, means, expressed in distance covered at US limit speed 55mph (88kms/h), that the person driving the car behind yours is going to see your indication 2,44 metres sooner. At a limit probability, imagine the difference between he/she managing to stop his/her car, let's say 44 cms far behind, or... 2 metres inside yours!!!!
That's why some cars are starting to incorporate their three brake lights made of LEDs