Oh this post has brought back all those memories....my DD would do the same and it used to send me crazy I was pulling over every few minutes so a drive any longer then 20 mins was out of the question for us....

Do you have some toys hanging so she can see them? they might distract her for at least a few minutes.

Do you have sun shades up? I know that my kids hated the sun coming in on them in the car
Is the seat belt to tight ?
Is the car seat comfy?
Could she be over heated? I know that my dd used to have to many layers of clothes on in the car and would get hot really easy...

If she is fed and you know there is no obviuos reason for her screaming then there is not much more you can do I guess.