Even if they do take a year or two to change from the Tetra radios, it's still a lot of money for something that will ultimately become redundant.
You also have the possibility of it detecting a Fire Engine, Doctor Car, Fire Car, Ambulance etc which may not even be on the same road as you (crossover bridges etc).
Add the fact it only works if the radio is transmitting, and you have to ask if it still represents good value for money.