the east-west highways are assigned even numbers for the most part. the north-south highways have odd numbers.
the odd numbers would begin at 1 (though there's not one lower than 5 AFAIK) and increase as they go east. so Interstate 5 runs along the west coast and Interstate 95 on the east coast. 'major' highways are assigned numbers that are multiples of 5 (5, 15, 25, 35, 55, 65, 75, 95, et al).
the even numbers would begin at 0 (there's not one) and increase as they go north. so Interstate 10 runs through the Southwest and the Gulf Coast and Interstate 90 runs through the great white north (*waves @ Bernie bros*). 'major' highways are assigned numbers that are multiples of 10 (10, 20, 40, 70, 80, 90, et al).
the US Highway numbering system is similar - except the odd numbers increase as they go west. so US Highway 1 runs along the east coast and US Highway 101 is out west (that's odd for another reason though). and then the even numbers increase as they go south. so US Highway 2 is in the great white north and US Highway 90 is way down south.