From the Manual on Uniform Traffic Control Devices (MUTCD), U.S. Department of Transportation, Federal Highway Administration, page 451:
B. Steady yellow signal indications shall have the following meanings:
1. Vehicular traffic facing a steady CIRCULAR YELLOW signal indication is thereby warned that the related green movement or the related flashing arrow movement is being terminated or that a steady red signal indication will be displayed immediately thereafter when vehicular traffic shall not enter the intersection. The rules set forth concerning vehicular operation under the movement(s) being terminated shall continue to apply while the steady CIRCULAR YELLOW signal indication is displayed.
Further down:
Section 4D.26 Yellow Change and Red Clearance Intervals
Standard: (page 485)
The exclusive function of the yellow change interval shall be to warn traffic of an impending change in the right-of-way assignment.
The duration of the yellow change interval shall be determined using engineering practices.
...
Guidance:
14 A yellow change interval should have a minimum duration of 3 seconds and a maximum duration of 6 seconds. The longer intervals should be reserved for use on approaches with higher speeds.