A 15-mile bike ride is being held in memory of 36-year-old cyclist Louisa Gag, a city transportation planner and bicycle safety advocate, who was struck and killed by a driver while biking on Tremont Street near Roxbury Crossing on July 9. Critical Mass Boston, a monthly cycling group, is planning a remembrance ride to honor Gag's contributions to bike safety in Boston. The ride will begin at Copley Square at 6:30 p.m. and will include free Bluebikes day passes and visits to Gag’s Bluebike stations, Boston Bikes Youth Instruction sites, Bike Towns and Adams Park. The event is also part of a demonstration to demand Mayor Michelle Wu better support the Streets Cabinet, which manages city roads, sidewalks, and traffic.