A first approximation calculation of the radiance or the retroreflected flux of light received by the automobile driver from a glass sphere partially embedded in highway marking paint is presented. The observed retroreflection arises mainly from the diffuse reflection at the painted region of the spherical cap on the rear surface of the sphere. It is found that the optimum value of embedment in paint for glass beads of refractive index 1.50 is 59% (+5%, -0%) of the diameter for high retroreflection at all distances.