The people in the outright coastal areas had to leave, but it is looking logistically impossible for the rest of Houston to do likewise.
Wrong. What we see on TV are the traffic stuck behind the choke points, where the roadways reduce to 2-lanes each way. Once beyond those points it can still be slow, but it will pick up. There is still a good 36 hours to evacuate from central Houston and north. Even if the traffic doesn't average above walking speed, that still gets the people leaving now more than 100 miles inland by late Friday night, which will buy them an extra 24 hours. Converting the outbound lanes will significantly add capacity. Anyone leaving within the next 12 hours will make it out of harms way IF they leave with a near full gas tank. But there are no guarantees if people wait until tomorrow.