Just Where Are All Those Empty Parking Spots At Dulles? Answer Man Hunts Them Down.

I fly frequently out of Dulles Airport, and I have a few burning questions regarding the airport parking garages. The garages have electronic boards as you enter the facility that tell you how many parking spaces are currently available on each parking level. The first question is, how do they monitor these spaces on a continual basis, as cars enter or leave the facility? But more important, why are the numbers so wildly off? Each time I've entered the garage, the board indicates, for instance, that Level 2 has 132 spaces. Upon arriving at Level 2, I am hard-pressed to find three spaces, let alone 132. The numbers are usually off by at least 100. What gives?

-- Terry McCaffrey, Leesburg

Answer Man realized that the only way to properly research Terry's question was to use his company credit card to buy a round-trip ticket to Paris on a flight leaving from Dulles. Then he decided that in these uncertain economic times -- when expenses are being scrutinized more closely than ever -- that might be a bad idea.

And so he just drove over to Daily Garage 1 and counted parking spaces. The lighted display said there were 20 empty parking spaces on Level 1.

Answer Man counted 55, not including more than a dozen handicapped parking spaces. The sign said Level 2 had 255 empty spaces. Answer Man stopped counting at 200. (Trust Answer Man on this: Counting parking spaces is boring.)

So, in Answer Man's limited experience, the counts were not off by a factor of 100, but nor were they completely accurate.

How does the system work? Dulles Airport's two daily garages use something called a ground induction loop to count cars coming in and out.

"Each night, a parking inventory is conducted by the contractor within those garages," explained airport spokesman Rob Yingling. "They count up the empty spaces on the levels and input that into a computer that generates the display. During the following day, when a vehicle enters or exits a level, it will trip a detection loop in the concrete that will change the space count display on each level."

So, if you start with, say, Level 2 showing 20 empty spaces, when a vehicle drives up to Level 2 it triggers the loop -- a wire embedded in the concrete -- and knocks the sign down to 19. If the vehicle exits Level 2 on its way to Level 3, it trips another loop, bumping the Level 2 display back up to 20 and subtracting a space from Level 3.

You can see where there might be problems. The system depends on an accurate human count every night. It also depends on vehicles behaving predictably, not behaving like Answer Man and driving all over up and down counting parking spaces. One parking expert said ground loop systems have a 60 to 65 percent accuracy rate. (The ground loop systems at two garages in downtown Silver Spring have been turned off because they were so inaccurate.)