Don't I love ellipses.

So the ellipse is horizontal. Its major axis, 2a, is 60 ft. Its minor axis, 2b, is 2*20 ft.

If the center of the ellipse is the center of the lane, then the ellipse is

(x^2)/(a^2) +(y^2)/(b^2) = 1

(x^2)/(30^2) +(y^2)/(20^2) = 1 -----(i)

At the edge of the lane, or at x = 20 ft, what is the y there?

(20^2)/(30^2) +(y^2)/(20^2) = 1

Clear the fractions, multiply both sides by (30^2)(20^2),

(20^2)(20^2) +(y^2)(30^2) = (30^2)(20^2)

(30^2)(y^2) = (30^2)(20^2) -(20^2)(20^2)

(30^2)(y^2) = (20^2)(30^2 -20^2)

(30^2)(y^2) = (20^2)[(30+20)(30-20)]

(30^2)(y^2) = (20^2)(50*10)

(30^2)(y^2) = (20^2)(500)

(30^2)(y^2) = (20^2)(10^2)(5)

y^2 = [(20^2)(10^2)(5)] / (30^2)

Take the square roots of both sides,

y = [(20)(10)sqrt(5)] / 30

y = 200sqrt(5) / 30

y = (20/3)sqrt(5) ft. ---------------answer.