Plans for a relief road around the M4 in Newport could lead to more crashes due to drivers falling asleep at the wheel, an inquiry has heard.
The Welsh Government's preferred option could result in a 4.4 mile (7km) detour to access the nearest services at Magor, Monmouthshire.
Roadchef, which runs it, told a public inquiry it would have "serious road safety consequences".
Welsh Government rejected the claim, saying drivers would be able to stop.
Officials have said the 14 mile (22km) six-lane motorway would relieve congestion between the current M4 junction 23A at Magor to junction 29, near Castleton.
But Roadchef has warned it could lead to an 80% drop in westbound custom, claiming people would not take the detour off the motorway.
This could lead to the closure of the services and loss of 216 jobs, leaving a "dangerous gap" in stops, written evidence submitted to the Newport public inquiry warned.
The Welsh Government has adapted initial plans to include the possibility of an eastbound slip-road at junction 23a.
Roadchef said this would allow the site to remain profitable, but said issues with westbound access would rule out any future investment and new jobs.
UK government advice says drivers should take a break of at least 15 minutes every two hours, with a maximum distance between service stations of no more than 28 miles (45km).
In written evidence, Dr Mike Axon, on behalf of Roadchef, claimed if people failed to leave the motorway due to the detour, the 16-mile (25km) gap between service stations, between Cardiff Gate and Magor, would become a 49 mile (78km) gap, with motorists having to travel to Leigh Delamere services.
He said it would create one of the "longest distances and times between stop rests on the UK's motorway network".
Roadchef chief executive Simon Turl said the lack of directly accessible services at the side of the motorway would "significantly increase the risk of major accidents due to driver fatigue".
He added: "The absence of proper toilet and rest facilities for travellers using the newly-built motorway access route into Wales should and would raise serious questions for all of those responsible for its construction.
"The spectre of desperate road users relieving themselves along the hard shoulder of the newest motorway in the UK would be highly embarrassing and extremely dangerous."
Speaking at the public inquiry in Newport, Rhodri Price Lewis QC, on behalf of Roadchef, said there was "overwhelming" evidence the current route would have a "significant accident risk".
He said an independent road safety audit found access arrangements to the services could result in crashes as people navigated junctions and took comfort breaks at the side of the road.
Mr Lewis added a westbound on slip road, costing £14.45m, should be built to stop motorists taking the risk of driving when tired.
In a written rebuttal, the Welsh Government said, while some road users "may be deterred" from using the services, "the reasonable road user with a compelling need to stop can and will be able to safely do so".
Roadchef's objections were, therefore, "matters of commercial impact rather than road safety".
It added creating an on slip road would delay building work on the relief road and would cost more.
The inquiry, which began in February and was expected to end in June, has faced numerous delays and will be adjourned next week until 5 November as the Welsh Government negotiates with the Association of British Ports over mitigation works to ease the impact of the relief road on Newport docks.
ABP are officially objecting to the compulsory purchase of about 87 acres of land to make way for the project.
It is hoped if an agreement is reached between the two, the inquiry would be finished by Christmas - if not it will last into the new year.