Billboard Advertising for Conferences?

I was driving in to the office a few months back and noted a particular local conference advertised on a prominent billboard leading into downtown Charleston, SC.

The only drawback is that I usually see these billboard advertisements up well past the conference date. This usually happens for the following reasons:

• Minimum time slot required - if so, we need a new negotiator. If I am the next advertiser in line for that particular billboard advertising space, I don’t care if my ad goes up on the 1st of the month or the 5th… so long as they prorate the cost.
• Branding purposes for perennial conferences - I would like to know how that works out. If you employ this strategy, tell us your experiences in the comment section below. I could think of better ways to spend the money, such as increased database segmentation of those who attended, enticing continued contact from previous participants, and using them for viral marketing (bring-a-friend discounts, find out what company they work for to contact their HR dept, etc)
• The advertising provider does not want bare billboards showing - I am not going to tell anyone what they should do with their company, but as an advertiser, I am more motivated by the opportunity - seeing an open billboard, thinking, “Hey, my company could advertise right there!”

I am curious as to how well these impact registrations for certain events. I will be keeping watch for more in my area, but if your organization experiments with billboard advertising for conferences, add any relevant data to the commments section below.