25 CFR §170.456

1. (a)A Tribe can request an exception from the required design standards from FHWA. The engineer of record (the State licensed civil engineer whose name and professional stamp appear on the PS&E or who is responsible for the overall project design) must submit written documentation with appropriate supporting data, sketches, details, and justification based on engineering analysis.
2. (b)FHWA can approve a project design that does not conform to the minimum criteria only after giving due consideration to all project conditions, such as:
   1. (1)Maximum service and safety benefits for the dollar invested;
   2. (2)Compatibility with adjacent features; and
   3. (3)Probable time before reconstruction of the project due to changed conditions or transportation demands.
3. (c)FHWA has 30 days from receiving the request to approve or decline the exception.