Laws regarding driving with a cracked windshield vary according to state. For instance, according to WOWT-NBC Omaha, it was legal to drive with a cracked windshield in Nebraska as of 2011, but doing so in Iowa could result in a driver being ticketed.
Centennial Glass points out that driving with a cracked windshield is illegal in most states. Doing so is dangerous both because of the possibility of the windshield breaking fully and because many windshield cracks impede a driver's vision. CarWindshields.info explains that states also differ in their requirements regarding windshield repair. Some states allow repair of cracked windshields with aftermarket parts, while others do not.