Unless you're throwing a neighborhood block party, almost all of the people coming to your event will have to travel there in some way. While you can't entirely control how people get around, you can give your attendees a nudge in the right direction.
Select a venue that can be walked to or that people can reach using public transportation. If public transport isn't an option, arrange group transportation from common pickup points. Even encouraging guests to carpool and share rides can help -- if you can reduce the number of individual vehicles to get people there, you're reducing your negative environmental impact. To help get people on board, consider offering incentives, like a free drink or a prize, to people who choose to arrive in ways other than by car. Lastly, don't forget to let guests know what the preferred transportation options are -- especially if they're not familiar with the area.