Two semitrailers will form a set of doubles. Which loading order does the handbook recommend for the safest handling?
Correct answer: Put the more heavily loaded trailer immediately behind the tractor.
The handbook favors the heavier trailer first and the lighter trailer behind it. Correct order does not replace individual axle, coupling and gross-weight limits.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Trailer loading order; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
A driver makes a sudden lane change with doubles or triples. Which trailer is especially vulnerable to the crack-the-whip effect?
Correct answer: The last trailer.
Steering movements can be amplified toward the rear of the combination. Steer smoothly and anticipate hazards; no fixed rollover probability is implied.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.1.2; Last trailer amplification; physical PDF page 200 (printed 200); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Why do doubles and triples require particular care on slippery roads and mountain routes?
Correct answer: The drive axles must pull more non-driven axles and a longer combination.
Additional trailers add length and non-driven axles. Slippery conditions can increase the chance of losing traction, so allow extra space and drive smoothly.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.1.6; Adverse-condition traction; physical PDF page 201 (printed 201); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Which parking plan is best for a doubles combination?
Correct answer: Choose a pull-through space with a manageable exit.
Choose a layout that permits a manageable exit. The length and articulation of doubles can make a confined parking area difficult to escape.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.1.7; Pull-through parking; physical PDF page 201 (printed 201); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
What is the role of a converter dolly in a common doubles combination?
Correct answer: It provides axles and a fifth wheel to connect a rear semitrailer to the lead trailer.
The converter gear supports and couples the rear semitrailer, forming a multiple-trailer combination. Its own coupling, tires, brakes and connections need inspection.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Figure 7.1; Converter dolly purpose; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
You are preparing to couple a rear trailer and are uncertain whether its brakes will hold. What should you do?
Correct answer: Use suitable wheel chocks and verify the trailer is secured before coupling.
Use suitable wheel chocks whenever brake holding ability is uncertain, and verify the trailer is secured before coupling. Level pavement or an assumed spring-brake system is not sufficient.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Rear trailer security; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
Before backing a converter dolly under the rear semitrailer, how should it be positioned?
Correct answer: In line with the rear trailer’s kingpin.
Straight alignment helps the dolly enter the rear trailer correctly. Verify height, security and the equipment-specific coupling sequence as well.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1, continued on page 203; Dolly alignment; physical PDF page 202 (printed 202); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After a converter dolly couples to the rear trailer, which checks does the handbook call for?
Correct answer: A gentle coupling pull and a visual check of correct plate contact and kingpin locking.
Verify the mechanical connection, not just electrical operation. The visual check must confirm proper engagement after the rig is safely secured.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Dolly coupling verification; physical PDF page 203 (printed 203); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After hooking the converter dolly to the lead trailer, what should be done with the dolly’s support?
Correct answer: Secure it in the raised position for movement.
The support is used when positioning or parking the dolly. It must be properly raised and retained for travel so it does not strike the road or obstacles.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.2.1; Converter support for travel; physical PDF page 203 (printed 203); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
After draining and completing the checks, what position is required for the converter dolly air-tank drain valve before normal operation?
Correct answer: Closed.
The drain must be closed to retain usable air. Draining removes moisture; leaving the valve open during travel creates an air-loss problem.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Dolly reservoir drain before travel; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
In a jurisdiction permitting triples: In a set of triples, what should be the normal travel position of the rear air-line shut-off valves on the first and middle trailers?
Correct answer: Open so service and supply air can reach the following trailers.
Intermediate valves must pass air along the combination. The unused valves at the rear of the final trailer are the ones that stay closed. Maryland does not permit triple-trailer highway operation.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Intermediate trailer valves; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
In a jurisdiction permitting triples: On the last trailer in a set of triples, no further trailer is connected. How should its rear service and emergency shut-off valves be set for travel?
Correct answer: Both closed.
The air lines end at that trailer. Closing both unused outlets avoids air loss while the connected upstream lines supply the rig. Maryland does not permit triple-trailer highway operation.
Source: Maryland Commercial Driver’s License Manual, DL-151 (02/26), February 2026; Section 7, 7.3.1; Final trailer isolation; physical PDF page 206 (printed 206); public mirror of the Maryland manual; reviewed September 15, 2026. ↗
