Teenage Daughter is a learner driver. When she first got her learners' permit, we had to take her out separately so she could drive on quiet streets. I used to worry about the emissions we were creating. Fortunately, now she is doing a lot better. She still has a long way to go to get her license but her driving practise can now be incorporated into other outings. For example, I wanted her to do a long drive on the weekend and intended for her to drive into the city and back. Husband needed to get to the airport and was going to order a taxi. It was a no-brainer really; forget the taxi and let Teenage Daughter do the airport drive instead of driving into the city. The airport is chaotic at present with a new parking lot being built so she and I simply swapped places just before we got there and swapped back just after we drove out.
It would be logical to assume that I wouldn't want to add another driver to the road; more emissions etc. However, most of the driving Husband and I do revolves around our children. We drive them somewhere, come home and go and pick them up a few hours later. Once Teenage Daughter has her license we'll actually half the emissions created when we drive her places because she can drive herself there, park the car and drive home a few hours later, thus eliminating the journey home in between that Husband and I make. That, of course, is assuming she doesn't enjoy her new-found freedom so much that she is constantly taking the car out and about!