Maybe everyone who obtains any driver license including 4 wheelers should have the same restriction? Just a thought.
In addition to what Errol points out, mountain driving is a factor. You must know how to manage gear selection in a manual big rig. You can blow an engine if you over-rev it, you can overheat your brakes if you rely too heavily on them at too high a speed, or you can rocket down a mountain out of control if you can't get it in gear. Those are not concerns for four-wheelers.
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Navypoppop proposes:
The difference between semi-trucks and 4-wheeler transmissions is the synchromesh feature. So the step between 4-wheeler and heavier vehicles is to learn how to manage the speed difference between each combination of gears (double clutching). The negative Auto only endorsement does that separation.
Double Clutch:
To engage and then disengage the clutch twice for every gear change.
When double clutching you will push in the clutch, take the gearshift out of gear, release the clutch, press the clutch in again, shift the gearshift into the next gear, then release the clutch.
This is done on standard transmissions which do not have synchronizers in them, like those found in almost all Class A trucks.
Double Clutching:
To engage and then disengage the clutch twice for every gear change.
When double clutching you will push in the clutch, take the gearshift out of gear, release the clutch, press the clutch in again, shift the gearshift into the next gear, then release the clutch.
This is done on standard transmissions which do not have synchronizers in them, like those found in almost all Class A trucks.