Rooftop cargo racks, otherwise simply known as roof-racks, provide an option for transporting cargo that does not use space inside a vehicle. However, despite the usefulness of roof-racks, once cargo is secured to a roof-rack on a vehicle there is the added potential for a collision between the cargo and some overhead obstruction—such as, for example only, a garage door, over-hanging sign, public parking structures, and tree branch. Previous collision avoidance systems that employ either ultrasonic acoustic or Doppler-based radar systems are poorly suited for detecting the potential of a collision between rooftop mounted cargo and an overhead obstruction. By contrast, aspects of the present invention provide systems and methods for reducing the risk of a collision between rooftop mounted cargo and overhead obstructions by providing a more suitable radar-based means for detecting overhead obstructions and warning drivers and, if present, passengers in vehicles when there is a risk of a collision.