Abstract Around 1970, high numbers of traffic casualties among cyclists led to the creation of numerous local protest movements in the Netherlands. While activists employed protest strategies, their main interest lie in the way they exemplify a highly successful instance of “lay expertise”; the idea that users of a technology have a fundamentally different and valuable perspective on a technology than experts or system-builders. Specifically, cyclists claimed to be more knowledgeable about cycling conditions and safety than the state-employed engineers and traffic experts who built the roads and cycling path network. A key actor in this story is the Dutch Cyclists’ Union ( Fietsersbond ), a national platform of local action groups formed in 1975. These activists used the cycling experience of everyday utilitarian cyclists to compile maps and blacklists of locations where cycling was dangerous, unpleasant, uncomfortable, or otherwise discouraging. In doing so, they successfully claimed legitimacy as a valuable knowledge partner for local engineers and policymakers. As a result, they gained some level of influence within local governments, a relation which in the intervening years has only grown stronger. This case study shows how users can shape socio-technical systems bottom-up, and can therefore to an extent be seen as a successful example of co-construction of technology.