After church, ariel and the girls go out for a milkshake, and she pulls that stunt on the road with chuck's car heading for the truck. What is obviously only minutes later in the story, both cars pull into the milkbar carpark and it is full dark out. Isn't church a Sunday morning thing?
OK... I know this gets a little technical, but it can explain why. Most churches have at least an early service and a late service. Most late services end between noon and 1:00, and sometimes even later. She could have easily waited around for a while after the service for her father to finish up and then drove out of their small town to a nearby town. I think the movie either started at the end of the summer or at the beginning of the school year... which is pretty much the same thing... so it would get dark earlier in the evening due to day light savings. Where I live, it can get dark as early as 6:00 pm after daylight savings and it can easily be an hour drive to my town from the smaller communities around. If she goofed off a little after church, it could very easily already be dark once the meet up at the drive in diner.
There is no way that the trip to the "milkbar" took hours to get to. Her dad (Shaw) got there like minutes after with money that her mother didn't think she had. Later on in the movie, Rusty meets her at the "milkbar" and it didn't take her hours to get there to meet Ariel.