I'm all for the cause of WWF, and I get the need to involve the regular Juan in their efforts to conserve our precious environment but interrupting the lives of people on their way home from a long day of work is NOT the way to do it. Interruptive Marketing never works. No one likes being stopped in the middle of a brisk walk by a total stranger. No one likes it when a sales agent suddenly gets in your face, with that stretched and worn-out smile. Heck, no one likes receiving spam. And you are SPAM PERSONIFIED!
I bet you guys close ZERO sales in your half-day stint in the underpass. Its because no one likes what you're doing! And I bet, you don't like it either.
There are better ways to get people to "buy-in" your cause. Sponsor a race, get runners or bikers to sign-up to save the environment, say the proceeds will go to the WWF fund, then SET-UP your booth there! That way, you've already narrowed your market to the people who actually signed up for the interruption.
Never ever be the irritating fly on the wall. Not when you've got such a great cause behind you.