Traffic speed variance is defined as a measure of the dispersion of space mean speeds among drivers. Empirical speed-density observations exhibit a structured traffic speed vari-ance which has been found to be associated to the road accident rate, the fatality rate, and travel time variability. The objective of this paper is to propose a generalized traffic speed variance function to describe this structured variance and identify its potential applications. In nature, the proposed speed variance function is a response of the speed-density curve with two additional parameters. A series of logistic speed-density curve with varying parameters is used in the proposed traffic speed variance function with different performances. This traffic speed variance model will help to unveil the underlying mechanism of some empirical traffic features such as spontaneous congestion or capacity drop.