Smith, K., Scallen, S.F., Knecht, W., & Hancock, P.A. (1998). An index of dynamic density. Human Factors, 40(1), 69-78.
The risk of a collision between aircraft is rising as the density of commercial air traffic increases. This trend, together with the overwhelming need to upgrade the National Airspace System, has motivated the Federal Aviation Administration to sponsor the development of metrics to evaluate “dynamic density”–a proxy for the likelihood of collision risk.