What is the minimum size for a Stop sign? Can I use 24 inch signs I have in stock?
The minimum size of Stop Signs (R1-1) is a 30 inch octagon. As for the 24 inch size signs, they are not allowed after January 2013.
Table 2B-1, Regulatory Sign and Plaque Sizes, from the 2009 MUTCD gives the minimum size for a stop sign on a highway as a 30 inch octagon. The compliance date for the new sizes is December 22, 2013 according to Table I-2, Target Compliance Dates Established by FHWA. This date was established by the changes in the 2003 National MUTCD.
While Table 2B-1. Regulatory Sign Sizes from the 2003 National MUTCD had a minimum sign of Stop signs as a 24 inch octagon, this smaller size was supposed to be used only on low-speed, not low-volume, roadways. The 30 inch size has been the standard for all conventional and low-volume roads since before 2001. The 24 inch size was only applicable in limited low-speed situations.
If your agency has 24 inch signs in stock, they may be able to use them on bicycle and shared-use trails that have no cars. Table 9B-1. Bicycle Facility Sign and Plaque Minimum Sizes shows the minimum size in this application as an 18 inch octagon. A 24 inch octagon would be acceptable. In fact, it might be advisable in places where not stopping the bicycle could result in a serious crash.