The Thing--which Severus calls an automobile, but the word "auto" always reminds Draco of sex acts he'd rather not discuss and so he will continue to think of it as "The Thing"--makes Draco nervous. It purrs like a Dragon on Pepper-Up--which doesn't make them happy, it makes them aggressive; MORE aggressive--and has none of the smooth ease of a broomstick. A broomstick in a rainstorm. With attacking seagulls.
(Draco has never had seagulls attack him, but he saw a picture of it once while he and Severus were walking through a Muggle street. The picture wasn't as clean as most wizarding ones are, and the store was displaying the same one in a variety of frames, which was a bit odd, but Draco remembers the birds. They were fright-inducing. He hadn't thought it was the way to get anyone to buy those frames.)
Severus says The Thing has its uses, and Draco supposes that with Severus not being able to apparate as much these days, and the floo system being hit and miss at best, well, it sort of does. He wouldn't do it at all, except that Severus always reaches over and pulls the strap from the door tight across Draco's chest and murmurs something about "safe, safe." The click of the mechanism locking into place has begun to feel like a promise, even if Draco can't figure out what the promise is.
He thinks he'll keep climbing in until he puzzles it out.