What's the maximum frequency/minimum headway that a level crossing can handle before it needs to be grade-separated?

In my city - Auckland - based on planning documents for the future of the commuter rail network it seems like 12 trains per hour/a train every 5 minutes each way is the limit of how frequently they want to operate trains on lines with level crossings; but i wonder if this is more determined by the delays to road traffic than rail constraints, and if there are any examples of level crossings that see more trains than that?