New research from Portland State University shows cyclists don't significantly reduce the speed of surrounding traffic, and the findings are universally applicable for how passenger cars are affected on urban roads without bike lanes. Looking at six Portland roads at all hours with speed zones under 40kmph with under 2000 vehicles per day, plenty of opportunities exist for motorists to overtake cyclists without overall speed reduction. Two sets of speed data were collected; from cars driving behind other cars, and cars driving behind bicycles. Actual speed differences between both data sets amounted to less than 1mph, without contributing to additional congestion.

The researchers concluded traffic control devices, traffic signals, pedestrian crossings, intersections with turning traffic and other road infrastructure factor in more significantly to travel time than just a few kmph difference in speed.

https://www.stuff.co.nz/national/300078096/do-bicycles-slow-cars-down-on-lowspeed-lowtraffic-roads