Transponders are definitely the best way to go.
I ran a RC car club for three years and using an Alley Cat software for all races was the only way to be absolutely acurate for timing and lap counting. The system gave a countdown, then start signal and counted individual competitors for the given race time, called you when you finished and calculated the winner by overall lap count and time right down to the tenth of a second. If you wanted detail individual lap times then it could be printed out after the race for you to make improvements for the next race. This system meant that it was driver skill, not grid position that won races.
The old days of someone counting you (hopefully) over the line each lap, and having to be relegated to a poor start position by luck of the draw have been left well behind.
If this was the system being used, then it wouldn't matter if you were a second after the start or 30 seconds as your 5 min race started when YOU crossed the line.
But what the heck, we're not playing for the farm, we're out there to have fun, make friends and go home with a bloody big smile on our faces.
See you there
Cheers Brett