| 3.5.3.1. | In addition, the stability in time of the optical properties and colour of retro-reflecting optical units of advance warning triangles conforming to an approved type and in use shall be verified. In the event of a systematic deficiency of the retro-reflecting optical units of advance warning triangles in use and conforming to an approved type, approval may be withdrawn. A "systematic deficiency" shall be deemed to exist where an approved type of advance warning triangle fails to meet the requirements of paragraph 5. | a0c0 |