Several top teams on Friday pulled out of the Etoile de Besseges cycling race in France citing safety concerns over traffic management on the course for a second straight day.
The peloton dismounted just after midday when a vehicle appeared on a roundabout.
Several teams disagreed with the resumption of the race and returned directly to their buses.
"Our team is one of the several that have decided to retire from today's stage due to several incidents over motor vehicles being allowed onto the race course," the Soudal Quick-Step Pro Cycling Team said.
"We find this unacceptable and decided not to rejoin the stage as we prioritise the safety of our rides and staff."
Soudal's French rider Paul Magnier had been leading the five-day race around Besseges in southern France.
The Ineos, Lidl-Trek, Uno-X and EF Education teams also ceased their participation.
A similar incident took place on Thursday provoking an accident which forced Belgian race favourite Maxim Van Gils of Bora-Red Bull to retire with a right hip and right wrist injury.
Cycling's governing body the UCI said it was investigating both incidents.
"Measures are being taken to avoid a repetition of the problems that occurred on these stages," read a statement.
"The UCI will continue to investigate thoroughly into the issues with the road closures, and take the necessary action bearing in mind that organisers are subject to... sanctions."
"The UCI emphasises that it takes the matter of rider security extremely seriously," it added.
Race organisers insisted they were "doing everything we can" in order to guarantee the safety of riders, saying it was not possible to fully privatise roads, as is the case in major races like the Tour de France, due to financial reasons.
"The car (that got onto the course Thursday), the police stopped it twice, then it drove off again. What can we do, apart from putting someone in front of each path?" said Claudine Fangille-Allegre, the Etoile de Besseges president whose father founded the race in 1971.
"We're volunteers. We don't earn anything. I'm getting to the point where I'm saying to myself, 'That's it, we've done 55.
"Either the riders understand that it's for them above all that we're doing it, or we won't be able to take on the burden of organising, and I'll understand that."
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