Well, it's natural to be 'stupid' in your dreams.
I've taken a ride with the CEO in a minature stealth fighter, through a city.
If that isn't 'Woah, that would never happen'.. anyway my point is it happens to everyone, so don't beat yourself up

Figure out your dreamsigns. Do you take a bus every day?
If not, be more vague and use 'buses and cars'
If you do see buses every day, then a more focused sign is better for now.

The point of dreamsigns and reality checks is twofold:
1) It raises your awareness of whether you're dreaming, at all times. It teaches you that you can easily fool yourself.
2) It provides an instinctive trigger for lucidity. This is important, because if it isn't a habit, and instinctive for you, then it probably won't happen (the lucidity that is)

Often people end up having a lucid dream without the dreamsign, but it was indirectly a help to them because it raised their emphasis and understanding of our sense of reality.
You just CAN'T expect to go to sleep and say 'If anything wierd happens, I must become lucid!'
That simply doesn't work. At least for most beginners