The following highways or routes are numbered 999.
MD 999 (former)
PA 999 
FM 999 | Preceded by 998 | Lists of highways 999 | Succeeded by 1000–1499 |
| 0–9 | |
|---|---|
| 10–16 | |
| 17–22 | |
| 23–31 | |
| 32–44 | |
| 45–63 | |
| 64–85 | |
| 86–103 | |
| 104–125 | |
| 126–149 | |
| 150–174 | |
| 175–199 | |
| 200–224 | |
| 225–249 | |
| 250–274 | |
| 275–299 | |
| 300–324 | |
| 325–349 | |
| 350–374 | |
| 375–399 | |
| 400–424 | |
| 425–449 | |
| 450–474 | |
| 475–499 | |
| 500–524 | |
| 525–549 | |
| 550–574 | |
| 575–599 | |
| 600–624 | |
| 625–649 | |
| 650–674 | |
| 675–699 | |
| 700–724 | |
| 725–749 | |
| 750–774 | |
| 775–799 | |
| 800–824 | |
| 825–849 | |
| 850–874 | |
| 875–899 | |
| 900–924 | |
| 925–949 | |
| 950–974 | |
| 975–999 | |
| 1000+ | |
