Hours of Service is the constraint that quietly governs dispatch. Plan against it badly and you get late loads, a driver stranded short of a receiver, or a violation.
The single most useful thing to understand is that HOS is not one limit. It is several clocks running at once, and the one that binds is whichever runs out first.
The short answer
A driver is subject to multiple simultaneous limits:
- A limit on driving time
- A limit on the on-duty shift — the window in which driving is allowed at all
- A limit across a multi-day cycle
- A required break after a period of driving
Drive time is the one everyone quotes, and it is frequently not the one that binds. A driver who spent hours at a shipper without driving has been burning shift time the whole while. Dispatch that plans only against remaining drive time will routinely be wrong.
The Federal Motor Carrier Safety Administration sets these rules for drivers of commercial motor vehicles in interstate commerce. The specific numbers have been revised more than once — check current FMCSA guidance for the values that apply to your operation rather than relying on what someone learned years ago.
Why "on duty" is broader than driving
This is where dispatchers most often get caught out. On-duty time is not just time behind the wheel. Waiting to load, inspecting the vehicle, fuelling, and paperwork are generally on-duty even though the truck is not moving.
The practical consequence: detention burns the shift clock. Four hours at a receiver does not just cost you money you may never bill — it consumes a chunk of the window in which that driver is permitted to drive at all. Two lanes with identical mileage can have very different feasibility depending on dock behaviour at each end.
This is why detention and HOS are the same conversation. A customer who reliably holds trucks is not just expensive; they are constraining what you can dispatch that driver to do afterwards.
The cycle clock is the one that surprises people
Daily limits reset with sufficient rest. The multi-day cycle limit does not work that way — it accumulates across a rolling window.
A driver can be perfectly legal on every individual day and still run out of cycle hours mid-week. Dispatchers who only look at today's clocks get blindsided by this, usually at the least convenient moment.
What this means for planning
A dispatch plan is realistic only if it accounts for:
- All the clocks, not just drive time
- Time at the dock, which is on-duty and often unpredictable
- The trip home or to the next pickup, not just the delivery
- Where the driver can legally and safely park when a clock expires
That last one is a real operational constraint and not a paperwork detail. A plan that runs a driver out of hours somewhere with nowhere to stop is a bad plan even if the arithmetic worked.
Records and enforcement
Duty status has to be recorded, and for most drivers that recording is done by an electronic logging device. See The ELD mandate, explained for what that device actually does.
Two things worth knowing:
Records are subject to inspection, and discrepancies between logged duty status and other evidence — fuel receipts, toll transactions, gate times — are the kind of thing that draws scrutiny. Consistency across your own records matters.
Falsifying logs is a serious matter for both driver and carrier. Dispatch pressure that can only be satisfied by a driver logging something untrue is a management problem, not a driver problem.
Where the software fits
The value of HOS data in a dispatch system is preventive: seeing remaining clocks before committing a driver to a load, rather than discovering the shortfall when they are already rolling.
TFS Fleet surfaces drive, shift, cycle and break-remaining clocks synced from your ELD provider, with violations shown first, and puts them alongside the dispatch board so a plan can be sanity-checked against the hours that actually remain.
See Fleet & Telematics and Dispatch & TMS.