Radial tires have been around for over 60 years. They are best used in areas where the weather is fairly moderate; minimal amounts of snow or rare instances of plummeting temperatures. They would have been ideal in areas like North Carolina, Ohio, Tennessee, north Texas, etc. I think radial tires were used a lot more than you would think back in the day. Drivers also used to put chains on their tires or use studded treads but they've been banned in a lot of places where there's more vehicular traffic, because they wrecked the road surface. They are still allowed in Northern Ontario, however, not sure about other areas with subzero temps.
I think the terms snow tires, winter tires, and radial all season tires kind of became interchangeable until the actual winter compound tires were produced in the early 80s.
Why are we thinking they were driving a van? Is it because it was kind of an iconic image for young people traveling the US? I haven't gone back to read earlier posts but I think there were measurements of the distance between front and back tires and between the front tires which might suggest they were driving a van but weren't cars back then as big as boats? I saw one last night outside a restaurant we went to, probably from the early 70s. I drive a SUV which can be considered a light truck and that different chassis decide whether a vehicle is a car or a truck. I read that in Ohio an Avalanche 'truck' is considered a car but in Florida it's licensed as a truck. Some are considered SUVs because they have unibody construction. That's all very confusing to me.