Assume the straight highway is along the x-axis.
Then the accessible locations of the walkie-talkie are defined by a circle of radius 1, centred on (102,0), as follows:
(x-x0)^2+(y-y0)^2=r^2
(x0,y0) is the centre of the circle = (102,0)
r=radius of range of equipment=1
so equation is
(x-102)^2+y^2<=1
in absolute-value form:
|x-102|<=|sqrt(1-y^2)|
where y is distance of any point from the highway.
