I searched and found a app for my phone that works great.
It uses the accelerometer on your phone to register a vibration as the RC passing by.
It was made for micros...put it on a ramp and every time the micro goes up the ramp it triggers the phone which records the lap # and the time.

But I put a 1/2'' thick board across my backyard track, with some of it sticking beyond the track border and put my phone on the board.(outside the track border) sure enough, it registered every pass and recorded each time. Shows your fastest and slowest lap when you hit finish as well. I was using a 1/12 scale eagle 3, which isn't heavy but made plenty of vibration for the phone to pick up the pass.

Its a free app. Wont work for racing head to head, but you can compete with your buddies for best lap or best overall time. Im sure success will vary by phone, but it worked great for me.

Its called "RC Race Timer".

Interesting. But you have to run over something to jostle the phone? Does that upset the car?

the board Im using is 1/2'' thick, so it doesn't upset the car much more than any other bump you would have on a race track. I may bevel the edges of the board, which would lessen how much it upsets the car.
Im going to make a box on the part of the board that extends beyond the track, which will protect the phone in case someone goes outside the boarder.

The video showing how the app works shows a simple cardboard box turned into a ramp for a losi micro truck, and the phone is on the ramp and reads the vibration. So, it may be possible to use an even thinner board than Im using. I would try, but the 1/2 thick board was the only one I had on hand.
Any 1/10 RC is going to jostle any board pretty good, enough for most phones to pick up the vibration.

Pretty clever, indeed.

ive wondered often why lap timing systems are so crazy expensive. Even just a transponder is a good chunk of change. We have cheap lipos, brushless motors, 2.4ghz radios...yet no one offers a budget friendly race timing system?

It would be cool if someone could make a sensor strip that plugs into your phone (phone used to record lap times, maybe not even plug in but wireless bluetooth) that came with 2 transponders (extras sold separately) for $100-$200. I think that would be reasonable for many who have a backyard race track.
This app is nice to track 1 models lap times, but what is really fun is to race against someone.

I am an industrial controls tech and have been playing with a few different systems to record lap times for multiple vehicles. Doing it for 1 vehicle is VERY easy and pretty affordable with just a laser eye and reflector. I haven't put a ton of time into it but if there is an interest in it I will definitely start a thread and track my progress and what I have tried.