When I went to traffic school here in phoenix a while back this came up. And the previous poster is right, you can drive up to 200 ft.
But the general consensus is you shouldn't try to get up to speed in it. If you are using the center lane to merge you should come to a stop until you see an opening.
It works pefectly fine that way. When people actually drive in that lane it makes things way too dangerous because someone from the other side could be trying to take a left.
It pisses me off when people drive in it. Yeah, sometimes there's heavy traffic and you really need to use it if you don't want to wait all day, but it's not that hard to come to a stop. I don't understand why anyone would want to drive in it when it's a fucking two way.