Bernard has a good plan there but...
In my experience of trying to solve this kind of problem, it is better to have a system/procedure in place that
forces the drivers to hand the radio back. An example, which wouldn't work here, would be that the driver swaps his/her car keys for the radio - they can't go home without their keys so they have to swap them back at the end of the visit.
You need to look at the process "in the round". If there is a gate and a gate house with a gate keeper, the gate keeper simply asks the driver for the radio before opening the gate. If there is a site office 200m before the unprotected exit (gate always open) then you're going to lose radios!
I once had a similar problem with access control cards - people took them home. We designed a "card-eating box" where the visitor dropped the card in a slot in the top, it slid down runners in the box until it stopped on a solenoid, in front of a short range reader. Once the card was read, the solenoid pulled in, allowing the card to drop into a collecting bin at the bottom and, at the same time, the exit door was unlocked. You couldn't get out unless you returned the card.
Maybe your best solution would be to do something similar: the exit gate (or barrier) would need to be electrically operated and usually closed. You could assemble some 4" drainpipe to act as a shoot with the top end up by the driver's window. Half way down, you could fit a microswitch (or something similar) that opened the gate. At the bottom, you could have a foam-lined box. The driver would simply drop his radio into the drainpipe to open the gate. Regular staff could use a little radio trasmitter fob or access control card.
This contraption would be fine if drivers are simply forgetting to hand the radios back rather than deliberatly stealing them! (If it's the latter, you'll end up with a box full of rocks!)
A few big signs saying "HAVE YOU REMEMBERED TO RETURN YOUR RADIO?" would be a good idea. You could even have one that lights up and flashes as the truck comes up to it...