Purple and White Roadside Flowers - Are they Phlox?

What is the name of the pretty purple and white flowers blooming along the freeways and natural areas? They bloom in May and look kind of like a phlox. They are pretty and must be easy to grow - where can I buy them?

These are Dame's rocket (Hesperis matronalis) a member of the mustard family and no relation to phlox. This plant is often sold as a wildflower but it is not native to the United States. Dame's rocket escaped from the garden and made itself at home along woodland edges and in meadows. I would not recommend planting it in the garden. It is a prolific seeder that can quickly take over the garden and add to the ever growing population already invading our natural areas. Enjoy the view from the car and look for another equally pretty, but less invasive plant, for the garden. For more information on this and other invasive plants visit the United States Department of Agriculture website www.plants.usda.gov for pictures and control options for this and other invasive plants.