That's because water gets inside the fissures and freezes, pressing against the surface. Come spring, dig out any infiltrating crabgrass and dirt, and fix the cracks for good. For asphalt, use a cold-pour filler, such as QPR Asphalt Crack Filler (about $11 at lowes.com
); for concrete, use Quikrete Concrete Crack Seal (about $10; acehardware.com
), then seal the whole driveway.