Operation Terror by Murray Leinster

The car was ordinary enough; it was one of those scaled-down vehicles which burn less fuel and offer less comfort than the so-called standard models. For fuel economy too, its speed had been lowered. But Lockley sent it up the brand-new highway as fast as it would go.

Now the highway followed a broad valley with a meadow-like floor. Now it seemed to pick its way between cliffs, and on occasion it ran over a concrete bridge spanning some swiftly flowing stream. At least once it went through a cut which might as well have been a tunnel, and the crackling noise of its motor echoed back from stony walls on either side.