The R21 is an important highway between Pretoria and the East Rand. It provides the only practical link between Johannesburg International Airport and the the Tshwane Metropolitan Area (Pretoria and Centurion), and is one of the only highways in Gauteng that remains relatively uncongested, even at rush hour.
There are minimum speed limit signs on many stretches that indicate that vehicles must travel faster than 60 km/h. There are also lane-use restriction signs that prohibit the use of the right-hand lane during rush hours by trucks (although when these signs are demolished in accidents, they don't seem to be replaced by the authorities).
However the minimum speed limits seem to be seldom enforced. Many trucks crawl along, sometimes at under the minimum speed limit. Today, a couple of Zimbabwe registered trucks (lots of trucks from that country use the highway for some reason) caused an enormous backup (and lots of hard braking) because they were traveling extremely slowly in the left-lane. There wasn't a traffic officer in sight.
The problems caused by trucks on the R21 need to be addressed by the authorities.