How to convince my parents to let me go on a road trip?

My cousins and I want to take a 6 day road trip from San Diego to San Fran for my 18th birthday. My mom and one of my cousins moms arent too sold on the idea. Theyre afraid of 3 girls being out alone, the dangers of driving, and such. How can I convince them to let me do this?!? Please help me out. Thanks :]