NASCAR Racing: scoring, electronic signals, nascar races

Question
My mom was a score keeper back in the day, when you wrote every car's # down in columns for each lap, as they went across the start/finish line. How are the Nascar races scored? Is there a sensor on each car and where is it located?  

Answer
Yes, there are electronic signals in the cars now and there are sensors located all the way around the track. NASCAR now uses those to reset the field from immediately after the yellow flag comes out. There is also one at the start finish/line to count laps and finishing positions.