Provincial transport MEC Donald Grant said Sidloyi (27) was flung from a BMW convertible on the N2 on Friday evening.
According to provincial traffic services, it was believed that three cars were involved in the crash on the N2 and that the BMW allegedly lost control around 8pm.
A video of the accident was posted online and showed the BMW clipping a car in the middle lane and then swerving over the fast lane into a concrete barrier wall.
Sidloyi was seen flung from the BMW before the vehicle bounced off the barrier, travelled across the lanes, and ended up next to the side of the highway.
"Early indications suggest that Sidloyi may have not been buckled up in the vehicle at the time of the crash. This is too often the case in a country with low compliance levels when it comes to restraints," Grant said.
He said road deaths were avoidable through responsible road use.
Excessive speeds, alcohol consumption and failure to wear a seatbelt remained the biggest problems.
"The Western Cape government extends its deepest condolences to Sidloyi's family and loved ones."
Sidloyi, from Gugulethu, was best known for his role in the 2012 Irish/South African film The Good Man.