Construction and bus workers union Unite, along with the train drivers union ASLEF, said sites must stagger start and finish times so thousands of workers are not travelling en masse on the same buses and tubes.
More than 20 transport workers in London have died from coronavirus, with the majority of victims working as bus drivers.
Unite secretary for the London and Eastern region Pete Kavanagh said: “It is imperative that construction firms stagger workers’ start and finish times.
“This should be happening on every site and employers should be liaising with each other, but also with oversight from industry federations, to make sure these times are different.”
Unite national officer for construction Jerry Swain added: “It is absolutely essential that they stagger the start and finish times for the thousands of staff due to begin work.
“This is not only imperative for London’s transport workers, who have already paid far too heavy a toll in the fight against Covid19, but for construction workers too.
“They, as well as their families and the general public, will also be at risk if they are forced to travel to work on crowded public transport simply because their bosses didn’t ensure staff were arriving and leaving at different times.
“We call on TfL and Sadiq Khan to work with Unite, construction companies and their federations, to outline a clear plan to stagger working times and to get workers safely to site.”