23 CFR § 655.604 - Achieving basic uniformity.
(a) Programs. Programs for the orderly and systematic upgrading of existing traffic control devices or the installation of needed traffic control devices on or off the Federal-aid system should be based on inventories made in accordance with the Highway Safety Program Guideline 21, “Roadway Safety.” These inventories provide the information necessary for programming traffic control device upgrading projects.
(b) Inventory. An inventory of all traffic control devices is recommended in the Highway Safety Program Guideline 21, “Roadway Safety.” Highway planning and research funds and highway related safety grant program funds may be used in statewide or systemwide studies or inventories. Also, metropolitan planning (PL) funds may be used in urbanized areas provided the activity is included in an approved unified work program.