Electric, autonomous buses and semi-trucks could have a huge impact
Tractor-trailers -- aka 18-wheelers -- are among the greatest factors in pollution caused by vehicles, both because they are allowed by law to pollute more, and because they accrue many, many times more miles than normal cars. The more electric trucks and buses we have on the highways instead of diesels, the better it is for our air.
Autonomy's role in trucks and buses cannot be ignored, either. Long-haul freight trucks, when operated autonomously, shouldn't need to stop other than for a malfunction or to recharge. Also, since a truck has much more space than a car, there's much more room for batteries, thereby extending the range. The result is the faster transportation of goods, which means reduced costs for retailers and manufacturers. That creates a demand for precisely the kind of truck Tesla's preparing to build.
That being said, Tesla is moving into this arena right at the same time as several other companies, so it's not without some very stiff competition. But at least Tesla's in on the ground floor this time, rather than playing catch-up as it is with passenger cars.