ATC: "Traffic 12 o'clock, 3 miles, same altitude opposite direction.
Turn 35 degrees right, I'll put you back on course when you're past him."
VFR Pilot: "Nah, that's ok, I don't see him so I should be fine."
Would you really do that? No? OK, let's just keep on flying then.
No, but the course of action, assuming you're not in airspace
where ATC has any responsibility for separation of VFR traffic,
is completely up to the pilot.
Order JO 7110.65T Air Traffic Control
Chapter 2. General Control
Section 1. General
2-1-6. SAFETY ALERT
Issue a safety alert to an aircraft if you are aware the
aircraft is in a position/altitude which, in your
judgment, places it in unsafe proximity to terrain,
obstructions, or other aircraft. Once the pilot informs
you action is being taken to resolve the situation, you
may discontinue the issuance of further alerts. Do not
assume that because someone else has responsibility
for the aircraft that the unsafe situation has been
observed and the safety alert issued; inform the
appropriate controller.
NOTE-
1. The issuance of a safety alert is a first priority (see
para 2-1-2, Duty Priority) once the controller observes
and recognizes a situation of unsafe aircraft proximity to
terrain, obstacles, or other aircraft. Conditions, such as
workload, traffic volume, the quality/limitations of the
radar system, and the available lead time to react are
factors in determining whether it is reasonable for the
controller to observe and recognize such situations. While
a controller cannot see immediately the development of
every situation where a safety alert must be issued, the
controller must remain vigilant for such situations and
issue a safety alert when the situation is recognized.
2. Recognition of situations of unsafe proximity may result
from MSAW/E-MSAW/LAAS, automatic altitude readouts,
Conflict/Mode C Intruder Alert, observations on a PAR
scope, or pilot reports.
3. Once the alert is issued, it is solely the pilot's
prerogative to determine what course of action, if any, will
be taken.
a. Terrain/Obstruction Alert. Immediately issue/
initiate an alert to an aircraft if you are aware the
aircraft is at an altitude which, in your judgment,
places it in unsafe proximity to terrain/obstructions.
Issue the alert as follows:
PHRASEOLOGY-
LOW ALTITUDE ALERT (call sign),
CHECK YOUR ALTITUDE IMMEDIATELY.
THE (as appropriate) MEA/MVA/MOCA/MIA IN YOUR
AREA IS (altitude),
or if an aircraft is past the final approach fix
(nonprecision approach),
or the outer marker,
or the fix used in lieu of the outer marker (precision
approach),
and, if known, issue
THE (as appropriate) MDA/DH IS (altitude).
b. Aircraft Conflict/Mode C Intruder Alert.
Immediately issue/initiate an alert to an aircraft if you
are aware of another aircraft at an altitude which you
believe places them in unsafe proximity. If feasible,
offer the pilot an alternate course of action.
c. When an alternate course of action is given, end
the transmission with the word “immediately.”
PHRASEOLOGY-
TRAFFIC ALERT (call sign) (position of aircraft) ADVISE
YOU TURN LEFT/RIGHT (heading),
and/or
CLIMB/DESCEND (specific altitude if appropriate)
IMMEDIATELY.
REFERENCE-
FAAO JO 7110.65, Para 5-14-1, Conflict Alert (CA) and Mode C
Intruder (MCI) Alert.
FAAO JO 7110.65, Para 5-14-2, En Route Minimum Safe Altitude
Warning (E-MSAW).
FAAO JO 7110.65, Para 5-15-6, CA/MCI.
FAAO JO 7110.65, Para 5-2-23, Altitude Filters.