The Manual on Uniform Traffic Control Devices (MUTCD) revised the minimum standards for retroreflectivity in 2012. The new minimum retroreflectivity standards are applicable for traffic signs but parking signs are excluded from them [Section 2A.08]. No Parking Signs must still meet other requirements in MUTCD (inspections, retroreflective, etc.). Engineer grade signs are likely to be acceptable for no parking signs. However, if your parking lot and roads are open to public travel, your signs must meet the same levels of retroreflectivity that are required for traffic signs on public highways and roads. Also, it is always a good idea to go for higher reflectivity even though the basic one might initially help you meet the minimum retroreflectivity levels. This is because the sheeting might quickly degrade to lower retroreflectivity level, thus losing its effectiveness at night and requiring replacement sooner.