I live in a rural area in California and only go to the land of HOV lanes, tolls, etc. about once a month.
I hate the look of the stickers are worry about damage to the car paint.
Someone in another thread proposed a removable sticker solution which I use. I can put the stickers on when needed and remove them cleanly when not needed. I use this material:
Amazon.com - Crafty-Fab by Vinyl Oasis - Self-Adhesive Craft -
I attach the sticker to this fabric which has an adhesive back and is "
Removable, Repositionable, & Reusable. Restickable up to 100 times! "
So far, it's worked well. It sticks well and removes easily. I always apply it to a clean surface and water doesn't seem to bother it.
This probably won't be a good solution if you need to use the HOV lanes frequently but for me, it's great.
(Note: This may violate some rule about applying the stickers "permanently" but I don't want a permanent sticker and nobody can tell the difference.)