Cant find any federal standards but this is CALI which is pretty picky (i think) when it comes to motor vehicles
http://caselaw.lp.findlaw.com/cacodes/veh/25100-25110.htmlCalifornia Codes
California Vehicle Code
VEHICLE CODE SECTION 25100-25110
25100. (a) Except as provided in subdivisions (b) and (d), every
vehicle 80 inches or more in overall width shall be equipped during
darkness as follows:
(1) At least one amber clearance lamp on each side mounted on a
forward-facing portion of the vehicle and visible from the front and
at least one red clearance lamp on each side mounted on a
rearward-facing portion of the vehicle and visible from the rear.
(2) At least one amber side-marker lamp on each side near the
front and at least one red side-marker lamp on each side near the
rear.
(3) At least one amber side-marker lamp on each side at or near
the center on trailers and semitrailers 30 feet or more in length and
which are manufactured and first registered after January 1, 1962.
Any such vehicle manufactured and first registered prior to January
1, 1962, may be so equipped.
(4) At least one amber side-marker lamp mounted at approximate
midpoint of housecars, motortrucks, and buses 30 or more feet in
length and manufactured on or after January 1, 1969. Any such
vehicle manufactured prior to January 1, 1969, may be so equipped.
(5) Combination clearance and side-marker lamps mounted as
side-marker lamps and meeting the visibility requirements for both
types of lamps may be used in lieu of required individual clearance
or side-marker lamps.
(b) The following vehicles when 80 inches or more in overall width
and not equipped as provided in subdivision (a) shall be equipped
during darkness as follows:
(1) Truck tractors shall be equipped with at least one amber
clearance lamp on each side on the front of the cab or sleeper and
may be equipped with amber side-marker lamps on each side.
(2) Truck tractors manufactured on or after January 1, 1969, shall
be equipped with one amber side-marker lamp on each side near the
front.
(3) Pole or pipe dollies, or logging dollies, shall be equipped
with at least one combination clearance and side-marker lamp on each
side showing red to the front, side, and rear.
(4) Vehicles, except truck tractors, which are 80 inches or more
in width over a distance not exceeding three feet from front to rear
shall be equipped with at least one amber combination clearance lamp
and side-marker lamp on each side visible from the front, side, and
rear if the projection is near the front of the vehicle and at least
one red lamp if the projection is near the rear of the vehicle.
(5) Towing motor vehicles engaged in driveaway-towaway operations
shall be equipped with at least one amber clearance lamp at each side
on the front and at least one amber side-marker lamp on each side
near the front.
(6) Towed motor vehicles engaged in driveaway-towaway operations
shall be equipped with at least one amber side-marker lamp on each
side of intermediate vehicles, and the rearmost vehicle shall be
equipped with at least one red side-marker lamp on each side and at
least one red clearance lamp on each side on the rear.
(7) Trailers and semitrailers designed for transporting single
boats in a cradle-type mounting and for launching the boat from the
rear of the trailer need not be equipped with front and rear
clearance lamps provided amber clearance lamps showing to the front
and red clearance lamps showing to the rear are located on each side
at or near the midpoint between the front and rear of the trailer to
indicate the extreme width of the trailer.
(c) Loads extending beyond the side of a vehicle where the overall
width of the vehicle and load is 80 inches or more shall be equipped
with an amber combination clearance and side-marker lamp on the side
at the front and a red combination clearance and side-marker lamp on
the side at the rear. In lieu of the foregoing requirement,
projecting loads not exceeding three feet from front to rear at the
extreme width shall be equipped with at least one amber combination
clearance and side-marker lamp on the side visible from the front,
side, and rear if the projection is near the front of the vehicle and
at least one red lamp if the projection is near the rear of the
vehicle.
(d) Clearance and side-marker lamps are not required on auxiliary
dollies or on passenger vehicles other than a housecar.
(e) Clearance lamps shall be visible from all distances between
500 feet and 50 feet to the front or rear of the vehicle, and
side-marker lamps shall be visible from all distances between 500
feet and 50 feet to the side of the vehicle.
(f) Clearance lamps shall, so far as is practicable, be mounted to
indicate the extreme width of the vehicle. Side-marker lamps shall
be mounted not lower than 15 inches on vehicles manufactured on and
after January 1, 1968. Combination clearance and side-marker lamps
required on loads shall be mounted so the lenses project to the outer
extremity of the vehicle or load.