When you hitch an unbraked trailer to a towing vehicle, the key legal and safety requirement (as used in UK-style driving theory questions) is: You must fit a secondary coupling device (such as a safety chain or breakaway/retaining cable).

What Must You Do When You Hitch an Unbraked Trailer to a Towing Vehicle?

The One “Must-Do” Action

For the question “what must you do when you hitch an unbraked trailer to a towing vehicle?”, the correct single action is:

  • Fit a secondary coupling device (safety chain or similar) to the towing vehicle.
    This keeps the trailer attached if the main coupling fails, helping prevent a runaway trailer and serious accidents.

Other options often listed in multiple-choice questions (like greasing the tow ball, turning off reversing sensors, or just checking mirrors) are good practices but not the specific mandatory requirement for this DVSA-style question.

Other Good Practices (But Not the “Must” in the Question)

While they’re not the correct answer to the specific theory question, safe drivers will also:

  • Check that the trailer’s weight and loading are within the towing limit of the vehicle.
  • Ensure lights, reflectors, and number plate are legal and working.
  • Make sure the hitch is properly locked onto the towball and secured with any latch/pin.

Think of the secondary coupling device as your backup safety net: the question is really testing whether you know that this extra physical connection is compulsory for an unbraked trailer.

Mini Story to Remember It

Imagine you’re towing a small garden trailer with no brakes for a weekend dump run.
You hit a bump, and the main hitch unexpectedly fails. If you do not have a secondary coupling, the trailer can shoot forward, hit other traffic, or cross the road on its own. With a secondary coupling device attached, the trailer stays connected long enough for you to stop safely. That’s why this is the must-do the test is asking about.

Quick Forum-Style Take

“Whenever you hook up an unbraked trailer, don’t overthink the theory answers. Grease and mirrors are nice, but the exam wants the life-saver: fit that secondary coupling device every time.”

TL;DR:
For the question “what must you do when you hitch an unbraked trailer to a towing vehicle?” the correct answer is:
Fit a secondary coupling device.

Information gathered from public forums or data available on the internet and portrayed here.