Are we allowed to put 12 inch high street signs (D3-1) on top of a stop sign (R1-1), or should they be put on a separate post?

Relative scale of stop and street signs with 12 and 8 inch blades

Street signs (D3-1) over stop signs (R1-1) are specifically mentioned as an example where the two signs can be on a single sign post. However, the street sign should not distract drivers nor obstruct the stop sign.

With a 12 inch blade, the cross street sign may be too large to be properly supported. Note that the 8 inch blade can be used for the cross street sign since a vehicle should be stopped. The street sign for the road with the stop sign needs to be 12 inches high unless the speed is 30 mph or less or the cross street also has to stop.

Therefore, the street sign for the road where the driver has to stop is generally on a separate post. The exception is when all traffic has to stop or in low-speed urban areas when all of the signs can be the smaller 8 inch blank.

The figure below shows the layout of a two way stop intersection with the larger sign for the higher speed traffic.

Street signs at a two-way intersection