Spur 399 begins where SH 121 branches off the Sam Rayburn Tollway, to merge with the Central Expressway.

After the interchange, the highway passes a few empty commercial building lots, before passing the Collin Higher Education Center to the north, and the Medical Center of McKinney to the south.

The road passes a small field, before reaching its eastern terminus, an at-grade intersection with SH 5.

[2][3] The freeway portion of the highway contains only one exit, for US 75 south, from the eastbound lanes, but the highway's frontage roads give access to and receive access from both directions of the Central Expressway.

TXDOT has announced a plan to extend this spur 6 miles to connect with US 380 east of McKinney.