Whoa, this looks promising! Thanks liudr. I'm going to check out their project with it and see if it will fit the bill!
liudr:
Very nice of you to get a project going with your dad. Good for you!As for hardware, neither arduino nor raspberry pi is required for a simple trigger. You can get this following module and hook the opto output to one of the trigger pins. You may want to add some small caps to smooth the rise of the signal so it doesn't trigger too many times. If you are unsatisfied with the result , get a small arduino clone on ebay, such as arduino nano, and program a debounce code and add a guard so when the sound is triggered, it will not be triggered again for a fixed time. Make sure it's long enough so subsequent cars don't all trigger the sound as they pass, unless that's your intention.
I don't have personal experience with this module but one of my clients vouches for it.