The topics of cars hearing other cars on duplex systems and of dedicated car-to-car channels are actually interesting. When the VHF high band system for the Los Angeles Police Department was set up the Chief of LAPD insisted that officers on patrol not have the ability to speak directly to each other. He was afraid that superiors could lose control of a situation if officers could bypass dispatch and speak directly to each other. Ultimately they established tactical channels ("1 Adam12, go to Tac 2"), but an officer had to go through dispatch to request a unit meet another on a tac channel. LAPD is now conventional digital UHF repeater with the repeater broadcasting dispatch and the cars. On the other hand, the LA County Sheriff's Dispatch channel repeaters transmit a beeping tone when an officer in the field is transmitting. Years ago when they were on lowband it was a telephone busy signal that let others know the frequency was busy.
Historically, at least here in the northeast, state police car to car communications were simplex. Up until the early 70's PA State Police dispatched on 42.62 and cars transmitted on 42.78. Cars could also transmit on 42.62 for car to car communications. When they went to high band the dispatch channels were duplex with no repeaters, and the original car to car frequency was 154.755. It was not repeated, but the bases stations could receive and transmit on this simplex channel. Not mentioned in this thread is that NYSP uses 154.665 for car to car. From monitoring it is clear troopers monitor this via the scan function.
The old VA State Police VHF system was similar to NYSP and the PA State police VHF high systems. I did discover a special feature many years ago while travelling there. While the tac channel was simplex,, troopers could request "Mobile Relay", a "by request" repeater allowing troopers to distant for simplex comms to converse with each other.
There are some advantages to not repeating a car to car channel. A repeated wide area system is always going to have dead spots, so two troopers close to each other in a critical situation might not be able to speak with each other even though they are very close. Secondly, if the tac channel for NYSP Troop C was repeated across the entire region only one conversation could occur at a time. With the current simplex channel troopers in Oneonta, Owego, Deposit and Cortland could all have short range conversations at the same time. There might be background static from distant transmissions but the short range transmissions would easily be received.
Finally, the Troopers in Broome and Tioga County have access to the repeated Sheriff dispatch channels in those counties. In fact, as the county communications centers answer all 911 calls, troopers receive most emergency dispatches over the Sheriff channels via polling so troopers hear the location of other troopers on those repeated channels.
