If both the inside and outside lanes on a superelevated horizontal curve slope down to the left, then it is a _____ -turning curve.

A superelevated horizontal curve is designed to help vehicles navigate turns more safely at higher speeds. In this scenario, if both the inside and outside lanes slope down to the left, it indicates that the curve itself is designed to facilitate a left turn. This is because the superelevation is added to counteract the lateral acceleration experienced by vehicles when they turn.

As vehicles approach a left turn, the banking of the road helps to keep the vehicles balanced by providing a force that assists in directing them through the turn, reducing the dependence on friction alone. In essence, a left-turning curve involves a horizontal curve that bends to the left, requiring the road to be superelevated in a manner that both lanes slope down to that direction. This design not only improves maneuverability but also enhances safety by reducing the likelihood of skidding outwards during the turn.

Understanding the correct answer helps in grasping the principles of roadway design and how they are applied in creating safe vehicular movement around curves.