Houston's highway system has a hub-and-spoke freeway structure serviced by multiple loops. The innermost loop is Interstate 610, which encircles downtown, the medical center, and many core neighborhoods with around a 8-mile (13 km) diameter. Beltway 8 and its freeway core, the Sam Houston Tollway, form the middle loop at a diameter of roughly 23 miles (37 km). A proposed highway project, State Highway 99 (Grand Parkway), will form a third loop outside of Houston, totaling 180 miles in length and making an almost-complete circumference, with the exception of crossing the ship channel. As of June 2014, two of eleven segments of State Highway 99 have been completed to the west of Houston, and three northern segments, totaling 38 miles, are actively under construction and scheduled to open to traffic late in 2015. In addition to the Sam Houston Tollway loop mentioned above, the Harris County Toll Road Authority currently operates four spoke tollways: The Katy Managed Lanes of Interstate 10, the Hardy Toll Road, the Westpark Tollway, and the Fort Bend Parkway Extension. Other spoke roads either planned or under construction include Crosby Freeway, and the future Alvin Freeway.
Try to answer this question if possible (otherwise reply "unanswerable"): What is the diameter of the center of Beltway 8?
23 miles