The through lanes of Massachusetts Avenue pass under Thomas Circle. The service lanes of Massachusetts Avenue intersected the circle until it was reconstructed in 2005–2006.
The circle is considered to mark the boundary between the downtown section of 14th Street and the emerging uptown 14th Street neighborhood.
Thomas Circle was originally implemented as a true traffic circle. Later, 14th Street was extended through the circle to increase traffic flow. In December 1938, construction began on the Massachusetts Avenue underpass; it opened for traffic on March 14, 1940.
In 2006, the D.C. Department of Transportation completed a $6 million reconstruction of the circle. The project included the addition of bike lanes, pedestrian crosswalks mid-circle (which hadn't previously existed), new in-circle traffic lights, better street lighting, and new sidewalks and landscaping.
See also 
|Wikimedia Commons has media related to: Thomas Circle|