After two years of driving the Roadster, I finally decided to put on my Clean Air Vehicle stickers. In California, they are only eight dollars and enable you to drive in the carpool lanes solo until Jan 1, 2015.
I am fortunate in that I rarely need to drive in traffic, but just started a weekly trip during afternoon rush hour. This trip without traffic takes one hour, but during the afternoon commute takes an hour and a half. The worst part of that drive has a carpool lane, so hopefully my trip will be faster now with the clean air stickers.
I avoided putting them on the Roadster because they are not easy to take off. I had read on the web that there is a 3M film you can put between the sticker and the car, which allows you to remove the sticker a lot easier. Unfortunately that film is only sold in giant quantities on the web.
The local dealership has something called Star Shield, probably similar to the 3M film, that they kindly put between the sticker and my car. The removal will require heat but the dealer will be happy to do that when and if the time comes.