Uniform standards for traffic control devices such as lane markings, traffic lights and stop signs are contained in the U.S. Transportation Department's Manual on Uniform Traffic Control Devices. The Highway Safety Act of 1966 requires stop signs and all other traffic control devices on public streets and highways to be in "substantial conformance" with the manual's standards.
Colors and Dimensions
Stop signs must be octagon-shaped with white trim and lettering on a red background. No other lettering besides "Stop" is allowed. If it's a three- or four-way stop, a plaque with the same red background and white lettering and trim can be placed below. The minimum size is 24 inches by 24 inches; the oversize dimension is 48 inches by 48 inches. Stop signs on surface streets must be 30 inches by 30 inches; on freeways, they must be 36 inches by 36 inches.
Stop Sign Installation
Federal regulations require certain conditions be met before stop signs can be installed. They are allowed if: a street is entering a through street or highway; a less-traveled road intersects with a main road and relying upon the right of rules would not be safe; an intersection lacking a traffic signal is located in area of signalized intersections; or restricted view, high speeds or traffic accident data indicate a need.
Stop signs are not allowed at intersections with functioning traffic signals. Portable stop signs are allowed only for emergencies and temporary traffic control. (Reference 3)
Stop Sign Placement
Federal regulations state that stop signs should be placed "as close as practical" to the intersection and where they are most visible. If visibility is obscured, a "Stop Ahead" sign should be installed. If a stop line is necessary as a supplement, it should be painted where drivers are supposed to stop. If the sign is installed at an intersection with an acute angle, it should be shielded or at angle where it is not visible to drivers on the other roadway. If the intersection has a marked crosswalk, stop sign placement should be prior to the crosswalk as close as possible to the oncoming traffic. If the intersection has two or more traffic lanes, another stop sign can be installed on the road's left side or a stop line painted. Channelized intersections can have stop signs in the channelizing island. (Reference 3)
Stop sign placement should be on the street with the lower traffic volume, but ultimately that should depend upon the traffic engineer's judgment. A traffic engineering study should support installation on a major street.
If two streets have equal traffic volumes or other characteristics, factors to consider for stop sign placement include whether a street has a longer uninterrupted stretch prior to the intersection and whether a street has bumps or dips already slowing down drivers. Other factors include a school walking route, pedestrian traffic and sight distance for cross traffic.
Not For Speed Control
Stop signs should not be used for controlling a driver's speed. According to the federal traffic control device manual, numerous studies show drivers' speeds increase after the signs are installed.