eliminate all trace of yellow used to indicate a restricted proceed condition. Lemon Yellow (The AAR's official name) - Used in position light systems as an all-purpose high visibility color, greatest fog penetration. (Plain) White - Plain incandescent white light. Used in dwarf position light signals with frosted lenses. Individual signal heads may be set to flash a color to create a different signal aspect. Signals in the United States typically flash only one head at a time, while signals in Canada may flash two heads at a time; flashing lights are generally less restrictive than steady lights. A few rapid transit systems utilize just two signal lamp colors (lunar white for proceeding and red for a full stop); examples include the Baltimore Metro SubwayLink, the Washington Metro and the PATCO Speedline. Speeds Signal rules and aspects make use of several pre-defined speeds. These speeds are also used in Weak Route type signaling. Normal Speed - The normal speed for the railroad line, also known as Maximum Authorized Speed (MAS). Limited Speed - A speed less than Normal Speed that was employed starting in the 1940s for use with higher speed turnouts (switches). This speed is defined by individual railroads and ranges anywhere from to . Medium Speed - Original concept for a standard "reduced" speed normally set to and can range as high as . This is the typical speed for diverging movements through interlockings and is also the speed trains are limited to when approaching a Stop or Restricted Proceed-type signals. Slow Speed - while within the limits of an interlocking and 20 mph when not in the limits of an interlocking. This is used for trains negotiating complex trackwork at interlockings. Restricted Speed - Used for trains entering or operating in unsignaled territory or when entering a de-energized track circuit. Regulatory definition of no greater than outside interlocking limits, 15 mph within interlocking limits. Trains operating at restricted speed must be able to stop within half vision short of any obstruction, and must look out for broken rails. Fault tolerance Signal aspects are designed to incorporate some degree of fault tolerance. Aspects are often designed so that a faulty or obscured lamp will cause the resulting aspect to be more restrictive than the intended one. Operating rules (GCOR, NORAC or CROR) require that dark or obscured signal heads be treated as displaying their most restrictive aspect (i.e.