A Bluetooth mouse + dongle costs $3. This is even cheaper.
The transponder would broadcast at 3 different AM frequencies, each frequency is for a different range, ~10 m, ~30 m and ~50 m, depending on conditions.
Different EM wave lengths reflect / refract around curves and through wooded areas differently so select the wavelengths accordingly.
A conventional AM radio would only be able to tune in to one frequency and the driver might want the longest range alert.
A dedicated receiver could be set to the short distance frequency in the city and/or the longer distance frequencies at higher speeds. Or it could be set to receive all three.
Even a drunk driver might sober up a bit hearing one or two alerts.
http://www.foxnews.com/us/2018/06/11/latest-man-who-killed-5-cyclists-gets-40-year-sentence.html
8 years/life.
Bret Cahill