I'm not sure if these are what you are seeing but, two white flowering shrubs that I have seen planted along roadways are Cordias. One is called Cordia Boisseri and it has a larger leaf and flower. The other one is a Little Leaf Cordia and is more "airy" looking with smaller flowers. Here's a link for the little leaf one.

http://www.delange.org/Cordia/Cordia.htm

thank you agility mom, I think you might be right. Cordia is probably what they are- I wasn't familiar with this genus before.