After I read about your logical concern I hit Google to see if there was some sort of roof mounted vertical laser (or other technology) clearance detector, which I think would be cool if the cost was right. Unfortunately there was nothing that came up. There are plenty that attach to bridges, tunnels, etc. that DOTs use but nothing for a personal vehicle. I guess it would be an expensive mod just to have a CravenSpeed stubby.
IF we had backup sensors like on my Explorer, a man could rig an extra sensor pointing up. But I don't think we have them, do we?
Falcon