Using mass-transit systems such as trams or trains reduces the overall footprint of an event significantly. Providing busses and other mass transport options helps reduces the volume of fuel per person required to transport customers to your site. And if audience members still need to drive, 'car-pool', by offering spare seats in your car. Its better for the environment and it can help pay the ever-increasing petrol bills!
At Peats Ridge we provide a bus service from the closest train stations, car-pooling and even bike "bus" for keen cyclists to ride escorted to the festival.
The lowest emission transport is good old-fashioned human power! If your event is close to its audience see if you can encourage attendees to walk or bike to the event. Perhaps you can offer incentives to encourage your audience to use alternative transport or use a tax system to change culture.
At Peats Ridge we use a levy on every vehicle. This not only encourages people to travel together but also provides a pool of funds that can be used for site remediation and carbon sequestration projects.
For an excellent site to assist the organisation of car-pooling see our friends at http://www.thecarpool.com.au/.