MUTCD Section 2B.55 talks about ‘Photo Enforcement” signs clearly -
A PHOTO ENFORCED word message sign may be mounted below a regulatory sign to advise road users that the regulation (like speed limit) is being enforced by photographic equipment.
A TRAFFIC LAWS PHOTO ENFORCED sign may be installed at a jurisdictional boundary to advise road users that some of the traffic regulations within that jurisdiction are being enforced by photographic equipment.
Setting speed limits “for roads open to public travel” in the United States has always been the responsibility of state and local governments. Various engineering and traffic studies are conducted for a long time to set speed limits.
Every state has a basic speed statute that requires drivers to operate their vehicles at a speed that is reasonable and prudent for conditions. This basic rule is contained in the Uniform Vehicle Code (UVC) of that state. Maximum posted speed limits are usually set at a higher value for rural roads as compared to urban roads.
However, the government does not decide speed limits for “roads within private gated properties, except for gated toll roads, where access is restricted at all times, parking areas, driving aisles within parking areas and private grade crossings.”
Minimum Speed Limit Signs help maintain safety by keeping all traffic moving at the same pace. It can become dangerous if there is a difference in the speeds of vehicles moving on the same road. The more significant the difference in speeds, the higher the chances are for damage.
This also applies to vehicles that can’t maintain highway speeds like bicycles, go-karts, farm equipment, heavy equipment haulers, and anything else that can’t maintain speeds like other vehicles. Law enforcers can ticket those that “impede the flow of traffic”
According to the Table of “Sizes of regulatory signs” included in MUTCD Chapter 2B, the minimum size required for a Speed Limit Sign must be 18” X 24”. This sized sign is visible from a distance of 240 feet. On conventional single lane roads, the MUTCD Speed Limit Signs should at least be 24” x 30” (visible from a distance of 300 feet), while on multi-lane roads, it should be 30” x 36”. On expressways, the size should be 36” x 48”, whereas, on freeways, the Speed Limit Signs must be 48” x 60”. All these sizes are width x height.
For the Truck Speed Limit, Night Speed Limit, Minimum Speed Limit, and Combined Speed Limit refer to Table 2B-1.
For private roads like indoor parking lots and driveways, however, you can also use a 12” x 18” Speed Limit Sign which is visible from a distance of 180 feet.
The Manual on Uniform Traffic Control Devices (MUTCD) requires all speed limits to be in 5 MPH increments. Although there is no confirmed reason behind this decision, it could be because, for a long time in the U.S., most speedometers had index marks only on 5 MPH increments.
Also, 25, 55, and 65 have always been important numbers in the American speed limit history. These values were very popular and would have been considered as important factors for speed limits in multiples of 5 because:
25 is the default speed limit for residential roads in many states.
55 was the National Maximum Speed Limit set by Congress in 1973.
65 was the speed limit allowed by Congress in 1987 for selected rural freeways before the National Speed Limit was lifted in 1995.
State legislatures establish maximum speed limits according to road class (e.g., Interstate highways) and geographic area (e.g., rural vs. urban areas). These are the factors taken into consideration while determining the speed limit for a road -
The FINES HIGHER Sign is used to warn road users when increased fines are imposed for violating traffic rules like exceeding the speed limit within designated roadway segments. Fines Higher Signs must be posted where some kind of construction or maintenance work is going on or where the roadway, shoulder, or other conditions, including the presence of a school, require a speed reduction or extra caution. FINES HIGHER plaque shall be mounted below a regulatory or warning sign in a temporary traffic control zone, a school zone, or other applicable designated zone.