Runway end identifier lights (REIL^[1]) (ICAO identifies these as Runway Threshold Identification Lights) are installed at many airports to provide rapid and positive identification of the approach end of a particular runway. The system consists of a pair of synchronized flashing lights located laterally on each side of the runway threshold. REILs may be either omnidirectional or unidirectional facing the approach area.^[2] They are effective for:^[2]

• Identification of a runway surrounded by a preponderance of other lighting
• Identification of a runway which lacks contrast with surrounding terrain
• Identification of a runway during reduced visibility

The International Civil Aviation Organization (ICAO) recommends that:^[3]

• Runway threshold identification lights should be installed:
  • at the threshold of a non-precision approach runway when additional threshold conspicuity is necessary or where it is not practicable to provide other approach lighting aids; and
  • where a runway threshold is permanently displaced from the runway extremity or temporarily displaced from the normal position and additional threshold conspicuity is necessary.
• Runway threshold identification lights shall be located symmetrically about the runway centre line, in line with the threshold and approximately 10 meters outside each line of runway edge lights.
• Runway threshold identification lights should be flashing white lights with a flash frequency between 60 and 120 per minute.
• The lights shall be visible only in the direction of approach to the runway.

• FAA Aeronautical Information Manual