So Many Police and No Traffic Enforcement

I don't get it. I just did a quick count of the different police forces patrolling Harris County and came up with this list:

1. HPD
2. Harris County Sheriff
3. Metro Police
4. Various ISD Police
5. Various Constables
6. Texas State Troopers

I'm sure I'm leaving someone out but my point is that, with all of these police forces, why don't ANY of them seem to enforce traffic laws?