I could compare rubbing a lamp to taking the subway in environmental terms and the lamp comes out way ahead. It just takes more walking to get places with the lamp method.
If you do wish to compare two things, you probably should include the cost of your time and the environmental impact you would have during that amount of time.
You also want to consider the actual impacts of building the transportation systems, whether its rail or road.
It would be infinitely easier for you if each process involved were taxed according to its impact. Then the costs would be included in the price you pay and you could decide according to what works best for you.
Further, if self driving vehicles are well coordinated, they could achieve higher utilization of roadways, which would require less land and less concrete and steel to carry the same volume of people. Also, they would potentially be able to operate with more optimal acceleration and breaking.