How do I keep a crack from spreading?

Temperature is the biggest factor. Park in a garage or in shade when you can, crack a window slightly on a hot day so the cabin doesn't turn into an oven, and warm the car gradually instead of hitting the defroster at full heat on a frozen windshield.

Avoid flex. Potholes, speed bumps, railroad crossings and a hard-closed door all twist the body, and a windshield is designed to move a little with it. Drive gently until the damage is addressed, and don't lean on the glass while loading the car.

Then get resin into it. A repaired chip stops being a crack in progress; an untreated one is a crack waiting for the right cold morning. Store-bought kits work poorly on laminated windshield glass because they can't pull a vacuum on the break — the professional bridge does — so it is worth the short drive or the mobile visit.