Which of the following is explicitly named in 49 CFR 383.115(d) as a traffic-operation problem that doubles/triples combinations pose to other motorists?

49 CFR 383.115(d)

Steep grades are where a heavy, multi-trailer rig loses the most speed, and that directly causes the longer passing times the regulation calls out — understanding this helps a driver anticipate frustrated traffic backing up behind them on a climb, and plan lane positioning accordingly.