I second the Rescue Remedy recommendation. It does seem to wear off in 2-3 hours, so you'll have to re-apply it. I also second the recommendation to keep the cat in the carrier.
Here is a paragraph out of my LJ entry on the cats, and the move:
As we were warned that Sammie was at special risk of running away in a panic, I made sure to get new harnesses and tags with updated contact information for them. We used retractable leashes to let them stretch their legs at rest areas. After a couple days on the road, things were going so well, that I thought I could handle taking both cats at the same time out for a little walk in the picnic area at a rest stop in Iowa. I carried them down to the furthest, loneliest ramada, and let them down to walk a bit. Suddenly a family with small children walked up to the ramada we were at. The children did what came naturally to them: They yelled, "Kitties!" and ran right at us. Before I had time to do more than lock down the leashes from extending all the way, Harvey's leash got caught in a bush, and he stopped running. Sammie took off, got to the end of her leash and began flipping out. She flipped and jumped and flopped against the leash so hard that her harness got loosened, and she got out of it, and started running. She was headed straight for the highway. I ran after her but there was no way I would be able to catch her without help. Thank Heavens for the observant people in the parking lot. A man who was just getting out of his car saw what was happening, opened his car door wide and stood in her way. In a split second she turned and was running up the hill the building with the bathrooms was on, and other people quickly formed a circle to stop her. She froze, and then I was able to get hold of her. Carrying her, I went back to get Harvey. Then I took them back to the van, thanking everyone we passed for their help.