By completing side missions, players slowly progress through the map's story to recreate iconic battles. (kotaku.com)
You can either complete all the missions in story mode AND complete the side missions with the best time; or destroy all the cars in destruction derby before time runs out. (gamerankings.com)
[19] The two players can explore the entire map and complete side missions separately, but main story missions must be completed together. (en.wikipedia.org)