State laws differ though most states in the US require that a windshield be free of any cracks or issues with the safety glass that would cause your vision as the driver to be impaired.
For example the Wisconsin Department of Transportation states that you can drive a car with a cracked windshield but it depends on where the crack is. It cannot be in the critical area (windshield wiper area). Wisconsin Transportation Code 305.34 is about windshields. Here it states:
(1) Every motor vehicle manufactured after January 1, 1936, shall have a windshield made of safety glass. Such glass shall be plainly marked to identify it as safety glass or a certification from the glass supplier stating that the glass is approved safety glass shall be carried in the vehicle during operation or vehicle inspection, or both.
(2) The windshield shall be of sufficient size to provide adequate protection for the driver and shall be in place when operating on a highway.
(3) The windshield may not be excessively cracked or damaged. A windshield is excessively cracked or damaged if:
(a) The windshield has a crack inside, or which extends inside, the windshield critical area.
(b) The windshield has cracks which extend into any area more than 8 inches from the frame.
(c) The windshield has stone or shot damage with a diameter in excess of 1/2 inch in the windshield critical area or in excess of 11/2 inches in diameter in any other area.
In Florida Section 316.2952 of the Florida Statutes, provides that a vehicle must have a windshield, and section 316.610 states that it is a civil infraction to drive a vehicle that is in an unsafe condition because of faulty or defective equipment that endangers the driver or other members of the public.
FL courts have held that it is a violation of this section to drive when the cracked windshield impedes a driver's vision. The size and shape or length of the crack is not specifically laid out in the law and thus up to the discretion of the law officer to determine if the crack impairs the driver's vision when driving.
To find out your state's specific laws regarding windshields and cracks or impairment of vision contact your state's Department of Motor Vehicles.