Go to World Cities in Multiplayer and search for the creator "db dazzler db" click on the "EZ Litterbug" city and choose EZ Litterbug. This track is a long 1 lap circuit on the highway with hedges placed throughout on the left lane. If you run over these hedges throughout the whole track then you will end with around $25,000 of collateral damage in a single run which will unlock the Litterbug achievement.
Repeat this event as many times as you need to reach a total of $100,000 worth of collateral damage. You can check your total collateral damage progress from the main menu by selecting View Progress> Player Statistics.