Some intersections with traffic lights have special bike facilities to help speed light changes when cyclists are waiting at a red light where there are no cars (which have their own signal detection loops that bikes don’t trigger). These are called loop detectors and they are coils of wire set into the pavement. When the metal parts, such as wheel rims, of a bicycle are detected electromagnetically, the loop detector tells the light control mechanism that a bike is waiting. The light may not change immediately, but the sequencing will take into account that a cyclist is waiting.
Loop detectors are usually behind the white stop line and generally slightly to the right. The older street marking that indicates a loop detector is a T as in the photo on the right. Newer markings conform to a national standard. It is a line with a bike embedded in it – see photo on left. In both cases, position the bike’s front wheel over the marking.
Some intersections don’t have this marker, or it may be faded away. If you think a specific intersection should have a marker you can contact walkandbike@seattle.gov to let them know.
I have seen the “t”s and wondered and now I know.