You can minimise your car's environmental impact by choosing a route that will allow you to make as few stops as possible and to maintain a steady speed.
The shortest route is not always best for the environmen. Choosing a shorter route to your destination may actually be worse for the environment if the shorter route means you will encounter more traffic, queues, junctions and traffic lights than on the longer route. In order to save fuel, it is important that you make as few stops as possible and that you maintain a steady speed.
For example, if you can choose between driving through a large city or driving around it, it is often more fuel-efficient to drive around it.