DALLAS — A makeshift bike lane divider made of painted two-by-fours and PVC pipes lasted three days on a busy Dallas street last month before the city removed it, which was two days longer than its creators expected. The $100 structure was the work of the Dallas Transformation Department, one of several like-minded groups of anonymous Twitter users who have taken a do-it-yourself approach to making road improvements in cities stretching from New York and Boston to San Francisco. Activists say a flower planted in a pothole or a line of cones or toilet plungers to keep cars from drifting into bike lanes can have the magical psychological effect of getting drivers to slow down and watch for cyclists and pedestrians.