bitpicker,
Well, but... then such a situation: my friend from England has arrived in my city for the first time, he call me...

he: -Hi oldboy! It's Harry. I'm at the airport now. Let's met.
I: -Hi! I'd love to. I will be waiting for you on the/a bridge in an hour. Try to catch a taxi and drove up to there.

Conditions is the same: that bridge is the only one in my city, but my friend doesn't about it (nor about the bridge itself, nor that it's the only one in my city). But if he ask a taxi driver to take him to the bridge, the driver can see where to go. So how should I act: say him "...on a bridge in an hour. it's the only one in the city, so If you ask somebody about it, they will explain you how to reach." or just say "...on the bridge in an hour."?