I am told that it's much easier to get University-business insurance coverage if you're driving a car rented for the purpose than if you're driving your own car. So...

Take the chartstring for the course or grant involved to 135 Giannini. There is a binder with forms to fill out where you're going, when you're expected back, your DL, and the chartstring. You get an ID number that Enterprise will recognize (UCB has a deal with them).

You can also arrange an ID number associated with a particular chartstring, or a gas card for your lab, if you have a research project that requires a lot of driving.