James Whitmore is stepping down from his role as managing director of Route due to health issues, after nearly 15 years at the helm.
Whitmore joined the audience research body for out-of-home advertising, as its first-ever managing director in 2005.
Route said he had been "instrumental" in making the organisation the world’s leading outdoor advertising measurement body and that a search for his successor would begin immediately.
Whitmore said: "For health reasons, I have made the decision to stop work. It will be sad to leave, but it’s an exciting time for the business and I am confident that Route will continue to grow under strong leadership."
Justin Cochrane, chair of Outsmart, the marketing body for OOH media, said: "James has massively advanced the measurement of OOH media in the UK and indeed globally, providing our members with both a granular understanding of how OOH is consumed and in bolstering the growth in the OOH market with its data."
Katherine Almond, chair of Route, said: "With the imminent release of the renewed currency, it is clear that Route will continue to deliver world-leading OOH research and analytics, setting benchmarks that allow the medium to flourish and grow.
"We will begin the process of finding a successor to James immediately. We want to build on his great work in creating not only Route, but also this latest iteration."