Dowsett took his fourth British title in the discipline, equalling the record held by Stuart Dangerfield.
The 26-year-old Movistar pro beat Edmund Bradbury (NFTO) by over three and a half minutes on the 45km course based at the Cadwell Park Motor Racing Circuit. Ryan Perry (SportGrub) placed third.
Dowsett's ride didn't all go to plan, however, as he left the tarmac on a tight corner and was forced to ride a section of grass. The incident did little to faze the 2015 Bayern Rundfahrt winner and he cruised to a comfortable winning margin.
Simmonds (Team Velosport) added to her 2014 national 10- and 50-mile time trial titles in the women's event, beating Molly Weaver (Liv-Plantur) by 1-18, with Dame Sarah Storey (Pearl Izumi Sports Tours International) in third.
Scott Davies (100% ME) had earlier successfully defended his under-23 men's title, beating Owain Doull (Team Wiggins) and Tao Geoghegan Harrt (Axeon).
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