What causes the white lines to crack on tennis and basketball courts?

Cracks on white lines are created by the expansion and contraction of the asphalt at different rates. The white lines, with their bright color, produce high reflectivity of the sun's rays. This results in cooler asphalt and minimal expansion under the lines. The surrounding playing surfaces have darker surface colors, which absorb more heat from the bright sun. In contrast, this tends to soften the asphalt binder and cause expansion. Over time, this expansion and contraction can lead to cracks in the asphalt (which loses flexibility and becomes more brittle as it ages).

Once cracks open up in the asphalt tennis court surface, the exposed asphalt begins to shrink and water intrusion can exacerbate the problem and increase the size of the crack at a quicker rate. Water settles into the stone base and wreaks havoc, especially in "freeze-thaw" regions. The additional expansion and contraction in the base will speed up degradation and the cracks continue to grow.