My husband and I have done a 16 hour road trip to visit my family in SC once or twice a year since my first was three months old.
At nine months old it can be very easy or very hard. Depends on your child. Here are a few things you might consider depending on your child:
1.) My oldest loved Elmo at that age (the other two could care less about tv). If you have a dvd player your little one can look at that might help.
2.) Sit in the back with your little one might help. With my son I think it makes him just want to get out and play with me, so I sit in the front. My oldest just liked seeing mommy.
3.) Finger snacks you can hand your little one to keep him/her busy...like cheerios. A sippy cup and some spares.
4.) Comfortable clothing like jammies.
5.) Does your car seat recline? That's nice when they fall asleep. I love those.
6.) A blanket for when they do fall asleep.
7.) Pacifiers or teething toys are nice if he/she will take them.
8.) Make regular stops to get out and stretch. Don't wake them if you don't have to. Travel as far as you can while he/she is sleeping and then stop at rest stops, gas stations for gas or a restaurant for a snack when they start to get crabby. Wait until they get crabby for your stops.
If he/she doesn't do well in a car seat now (is he/she turned around yet?) I know that most don't turn their children around until a year old, but mine hated being backward. When they faced front they did better because they can see.
If your little one is not good in a car, then I would seriously consider driving at night so he/she will sleep the whole way. That is if you and the significant other can stay awake for the drive. There are a few occasions when my hubby and I decided to drive all night so the little ones would sleep. It depends on your little person.
Good luck!! Don't let yourself get stressed if they cry. They will live. Make jokes, take comfort foods, and get some books on tape to listen to. I love books on tape and when the hubby and I finally don't have anything else to talk about we pop in a book on tape and listen to that.....we love the Anne Coulter books because she's such a sharp wit and has an amazing way of making an argument. I also like mysteries.
Have fun!