High Occupancy ?

What is the overall idea of why you should be able to drive in the HOV lane? This morning as we drove into work, my boyfriend, Tylan, and I had an interesting discussion about HOV rules. It all came up when I saw a motorcycle heading into the HOV only lane. Is the idea that they want "high occupancy vehicles" or less cars in Boston? Either way a motorcycle takes up less space and with only one person half its "occupancy" is full. But with that last comment, why aren't two "seater" cars whose occupancy is "high" aka "half," not allowed in the HOV lane also? Food for thought. Plus, it's very lovely to wake up in the mornings and have someone to commute with. It's even enjoyable when your partner is half asleep. (Just make sure the person most awake is DRIVING!)