I make the drive between Hou and DFW every day.
From my observation, DPS VHF P25 is the states' primary system...but for the units in coverage areas of large metro trunked networks, that has become the de facto standard. Most troops seem to now run APX multi band radios. One control head, but multiple bands/protocols on one seamless control head.
VHF is multicast with whatever the local system is. In my area, when dispatch transmits, it is multicast over the VHF DPS and local 7/800 system. But whatever system units respond on, it can only be heard over the same system. You can tell when field radios are transmitting over VHF because you can hear both sides over the VHF side (think air units).
As far as the more rural traffic, they also like to hang out on the local radio systems (even if it's plain old conventional FM). Since the implementation of the LTE mobile IP , voice traffic can be reduced in certain areas.
To monitor DPS, you have to monitor whatever the local regional/county system you are passing through (regardless if it's a simple VHF conventional repeater or one of the huge digital trunked networks Texas is famous for).Rest assured there is still PLENTY of voice traffic, it's just either patched to the local trunked or direct unit contact through local SO radio systems.
Example: When driving through Madison up through Leon counties I will hear very little on the "proper" assigned VHF channels. But listening to those same respective counties' channels I will hear DPS units check in, pick up calls and directly communicate with county dispatchers.
They 97just have more options available due to new multiband radio equipment and MDC usage.
I still hear plenty of traffic stops, 28's and 29's, and accident responses as we've always heard. Only thing that's changed is it's not as simple as years past where one could just scan the whole State channel plan and catch everything.
Another clue is if I see area assigned vehicles with a 7/800 mhz antenna I pretty much assume they are actually TXing on whatever the local flavor is, albeit through patched or multicast channels.