Unfortunately, there is some falsehood in the question itself: cars were not popularized post-WW2; rather, it was the 20s and 30s in particularly rural America. What post-WW2 really popularized was long distance traveling due to better road conditions and inter-state highway programs started under such people as President Eisenhower.

You see, America caused a massive boom in the automobile industry for two reasons: rural America was wider than most of Europe, thus needing longer range, cheaper transport, *and* Henry Ford's creation of the assembly line. See here: https://www.history.com/topics/inventions/automobiles

With the farmers of the states wanting the cars, and Ford processing them faster than any other car company, the horseless carriage quickly began to surpass the horse. WW1 also played a vital role, as numerous factories decided to stick to vehicle manufacturing after the war ended, and the war popularized the truck and car. See statistics here: https://www.scientificamerican.com/article/the-motor-vehicle-1917-slide-show/ Cars quickly became a center of life for many Americans, as their love for such things as the good old "Tin Lizzie" and it's cheap $300-or-so price was proven as the U.S. soon owned the majority of the world's cars by the end of the 20s (see article below).

The culture *did* change however, and in more ways than one. Above all, the ways of commerce and work changed acrossed the states. jobs boomed in the industrial cities as new jobs in car manufacturing opened, and more people could drive into the city from further away for work. On the contrary, shopping forever altered as the average American could now drive to a distant supermarket whenever they needed rather than pop by their derelict general store or order from a catalog. New consumer styles also ended many lines of work, such as the milkman or the guy who replaced the ice in the back of your fridge (yes, that was a job). Now, you could cut the middleman out and drive for whatever you needed. See here all of these changes: https://schoolworkhelper.net/automobiles-in-1920s-history-production/

On a more cultural note, the American Teenager and the realm of dating changed forever. Now, teens could go on dates far beyond the peering eyes of snoopy parents and family, and perhaps sneak off to practice kissing at one of the new cinemas. Automobiles offered a realm of independence and privacy for teens of all kinds, whether they were dating, hooking up in the backseat, or just driving around. Parents no longer held full dominion over their children, whom now sped off on escapades far from their homes. Also, as I aforementioned, the car was predominantly owned by rural people and farmers who scattered about the countryside. Now, what was once a series of isolated homes connected through sparse visits to public places or the occasional telegraph, was connected by a series of dirty paths and black, oil chugging automobiles. While this was possible with the horse, it became more prevalent with a machine that needed no rest and could travel much further in ideal conditions.

It's also worth noting that around now the term "jaywalking" appeared as cars began to become kings of the road, not pedestrians.

So no, Post-WW2 America didn't popularize the car: the Roarin' 20s did. What *did* boom was even longer distance travel acrossed the United States and better driving conditions due to plans by people like Dwight D. Eisenhower to improve road conditions.

 While the 20s, 30s, and 40s steadily saw a rise in paved roads all across the states, but saw constant delays due to the Great Depression (in which New Deal Agencies preferred small, quickly made roads in large quantities to provide jobs rather than trusty, durable roads like the interstates we know today) and later WW2. There was even the creation of such things as the Federal Highway Act of 1921, which put a demand on highways, and later the Miller Act of 1935 which guaranteed completion of public projects without extra taxes. These acts gave the public an idea that maybe better roads would be possible, but there was never a good time to formulate the kinds of roads we truly use today. See here: https://surety1.com/history-highways-eisenhower/

Then came Eisenhower and the Cold War.

Due to a fear of traffic jamming in the event of a natural disaster (or more particularly, nuclear armageddon), a shift in economic and public attention returned to the bettering of the roads, which were in even worse condition since they majorily fell into disrepair during WW2. So, along came the Eisenhower administration. You see, due to a poor road trip experience, fear from the 1953 Soviet successful testing of the hydrogen bomb, and his military knowledge in the value of efficient transportation, Eisenhower swiftly put focus into the IHS, formally known as the Dwight D. Eisenhower National System of Interstate and Defense Highways, see here: https://www.army.mil/article/198095/dwight_d_eisenhower_and_the_birth_of_the_interstate_highway_system

Highway and road praving would boom, and soon America was on wheels and looking on their maps not just for the grocer, but for the coast, landmarks, national parks, and other road trip destinations. Now, it was increasingly more possible to go see Elvis Presley's concert in New York, even if you live in Atlanta or Chicago. This, the road trip was born, all due to WW2 and fear of the atom bomb. 

Thus, Post-WW2 America was deeply influenced not by the popularization of cars (although the amount of cars taking to the roads was no doubt steadily rising), but rather by the creation of Interstate Highways and well paved and maintained roads, which improved road safety and made driving easier and swifter. Was etiquette affected by such a change? Most likely not, as the fear of car crashes still existed in these seatbelt-less vehicles, no matter how good the road.