“…Furthermore, he became interested in collision avoidance, starting with a working paper to facilitate discussion of the problem at an ICAO Airworthiness Committee meeting ( Calvert, 1958a ). At the time, collision avoidance at sea and in the air was a fiercely debated topic of great interest given that there were 159 mid-air collisions in the period 1948–1957 in the United States alone ( Calvert, 1960 ; see Sharpey-Schafer, 1955 and Wylie & Deacon, 1963 for ship collision statistics). Several mathematical treatments of the problem were developed ( Calvert, 1958a ; Morrel, 1961 ; Sadler, 1957 ), 27 which led to the development of several systems for collision avoidance ( Calvert, 1960 , 1973 ; Hollingdale, 1961 ; Wylie, 1960 ), investigations of the human factors aspects of the problem (e.g., Broadbent, 1958 ) and discussion about changing the rules of procedure at sea ( Calvert, 1969 , 1971 ; Kemp, 1965 ; Kemp et al, 1965 ).…”