A highway sign expert who reviewed a video showing the path that the bus took up the Northside Drive HOV exit off I-75 south said one sign was "potentially a killer."

The last overhead sign that HOV drivers see before the Northside Drive exit doesn't say "Exit." It says only "Northside Drive" and "buses/carpools only," and has an arrow pointing left and an HOV diamond.

"I was astounded at that sign," said Fred Hanscom of Washington, who sits on the panel that writes federal guidelines for highway signs.

There are signs before and after it that say "exit," but they are less noticeable, and the clearest one, pointing left at the mouth of the ramp, comes too late, he said. He called the signs approaching the ramp "ambiguous."

Hanscom's reaction came without visiting the site or doing a detailed study — he viewed the video on ajc.com. Hower, he said, the signs may not violate federal standards. A key issue is that drivers don't expect exits to be on the left, so signs here have to prepare them for that, he said.

When drivers' expectations are violated, he said, extra measures, like oversized stop-ahead signs with flashing beacons or pavement rumble strips, are recommended.

DOT spokeswoman Karlene Barron said even if drivers mistake the exit for the HOV lane, the ramp bears stop-ahead signs and stop signs.

NTSB board member Kitty Higgins said she was less concerned about that sign than "other things that might help" like a ramp speed limit sign.