Just got back from DCA last week. If you want to get FP's for Radiator Racers at all, you will need to get to DCA for the park opening.
There is an entirely separate line to the left of the main area where people with congregate for rope drop. This is a FP only line. You will want to send someone from your party into this line. It is very organized, and takes about 20-30 minutes for a person to get through the line for FP's. When we were there last week, park opening was at 8 AM each day. FP's were completely gone by 9 AM each day, and really only went to people that got in this line.
Other than the FP line, you can move with the crowd to Cars Land after rope drop. It's the usual cattle heard that you're used to, especially if you have been to DHS for Toy Story Mania at opening. We attempted to have the rest of our party get directly onto Radiator Racers from rope drop, but on both days, the wait was already up to about 60 minutes. Keep in mind that RSR is as slow loading as Test Track, so they can't just put a ton of people on at the beginning of the day.
If you don't mind waiting 60 min or more right from rope drop, then head right to RSR. On both days we were there, we opted out of that line and went on Luigi's Tires and Mater's Junkyard Jamboree instead. There was little to no wait at rope drop for these, and the both get very busy later in the day.