A sign with a yellow background and a number like 12'-6" and two black arrows pointing up and down is a _______ sign.

A sign featuring a yellow background with a number like 12'-6" accompanied by two black arrows pointing up and down is specifically indicating low clearance. This type of sign alerts drivers that there is a height limitation for vehicles, meaning that any vehicle taller than the specified height (in this case, 12 feet and 6 inches) may not be able to pass safely under an overpass, bridge, or other structure.

The use of the yellow background helps to enhance visibility and convey caution, which is vital in situations where potential obstacles exist. The arrows pointing up and down symbolize the spatial limitations that drivers need to be aware of when approaching the area.

In this context, while 'height restriction sign' could also seem applicable, the term 'low clearance' more accurately describes the particular scenario of height limitations related to passing through structures. The other options such as weight limit sign and speed limit sign do not pertain to height and thus are not suitable for this specific type of cautionary signage.