Table 1.

Definitions of symbols used to designate spatial, temporal, and optical variables.

Symbol	Definition	Figures
SPATIAL AND TEMPORAL VARIABLES
xo, zo	Position of observer along x and z axes, respectively	2A
xm, zm	Position of moving object along x and z axes, respectively	2A
W	Width of observer's body	2A
E	Observer's eyeheight	2B
Vmax	Maximum speed at which the observer is capable of moving	–
t	Current time	–
t*	Time at moment that leading edge of object first reaches the locomotor path (gray area in Figure 2A)	–
TTC	Amount of time remaining until the object crosses the reference axis	–
k	A parameter equal to one-half of the ratio of the observer's body width (W) to his or her eyeheight (E)	–
dfront	Minimum distance that the observer must travel to pass in front of the moving object	2A
dbehind	Minimum distance that the observer must travel to pass behind the moving object	–
vfront	Minimum speed at which the observer must travel to pass in front of the moving object	–
vbehind	Maximum speed at which the observer could travel and still safely pass behind the moving object	–
OPTICAL VARIABLES
α	Angle between reference direction and moving object	2B
θ	Angle subtended by edges of moving object	2B
γ	Angle of declination of projection of base of object along reference axis	2B