I also only go on field trips by car when I only have a small group, usually when it happens to be only 1 plus my own two. I have enough space in my suv for everyone, but not enough car seats and it is too much work to take all of them somewhere by myself. I have extra liability insurance on my car insurance and only very occasionally go by car for field trips. It is easy to do around the neighbour field trips where you can just walk, which the kids are just as happy to do.