How much more time and how many more miles would it have taken? How difficult would it have been to stay reasonably close to food, shelter, mechanical and medical assistance? What other challenges did drivers encounter?
Do you have a particular time period in mind? The first recorded automobile trip across the country took place in 1903, but there were a number of cross-country road projects between then and the Interstate system in the 1950s, such as the Lincoln Highway and the US Numbered Highway System (Routes): https://www.fhwa.dot.gov/infrastructure/numbers.cfm