To assist those scanner radio enthusiasts who monitor Route 21, this mile marker directory should assist you in determining which support agencies (Police, Fire, EMS) to monitor, should only a mile marker be given as the location.
| Mile Marker | Municipality | County |
| 0.0 - 5.6 | Newark | Essex |
| 5.6 - 7.6 | Belleville | Essex |
| 7.6 - 8.9 | Nutley | Essex |
| 8.9 - 10.1 | Clifton | Passaic |
| 10.1 - 13.2 | Passaic | Passaic |
| 13.2 - 14.35 | Clifton | Passaic |