If you check the link from which I copied that info, you will probably find it hot linked from there.
From Heritage Foundation-How many people were in Washington march?:
“1.5 million are reported to have passed something called the “people meter” [just] on Pennsylvania Avenue [alone]”
http://www.insideronline.org/blogarchive.cfm?month=9&year=2009#BA311C7A-962A-59E5-2F87892A97FB00CE
"Just got off the phone with Barbara Espinosa, who sent in tons of great pics from DC earlier today. Hopefully, we’ll have some video later, too. As a volunteer, she had “full access” to all parts of the protest. Here are some of the things she saw today, by the numbers.
•70 outdoor port-a-potties.
•90 minute wait to use one.
•The People Meter on Penn Ave had read 450,000 by noon, and 1.5 million two hours later. (That last number still looks awfully high to me.)
•1 DC police officer, who told her, “I’ve been here 20 years and this is the largest crowd I’ve ever seen.”
I could find no confirmation that there is any such thing as a "people meter" on Pennsylvania Ave.