An industry-backed autonomous car lobbying group on Tuesday urged US regulators to go further to remove speed bumps than what was laid out in guidelines published two months ago.

In a letter to the National Highway Traffic Safety Administration, a 'Self-Driving Coalition' founded early this year by Google, Ford, Volvo, Uber and Lyft hailed the framework but called anew for nationwide rules to speed the safe rollout of the technology.

The group asked the NHTSA to "affirmatively discourage states from embarking on creating a patchwork of inconsistent laws and regulations that will stifle this emerging industry."

Instead, the coalition contended, regulators should put in place a national framework of rules for self-driving vehicles. As published, the guidelines leave the potential for states to put in place overly restrictive or inconsistent regulations, the group argued.

The coalition also reasoned that fully autonomous vehicles promise the greatest benefits to safety, traffic and mobility so regulations should allow for vehicles designed to operate without human intervention.

The group urged the NHTSA to amend standards "to provide for the self-certification of vehicles that would allow fully self-driving operation without the presence of, or capability to use, human operator controls" such as steering wheels or brake pedals.

The coalition called for Congress to let the NHTSA allow larger test fleets of autonomous vehicles. Current rules limit manufacturers to no more than 2,500 vehicles, with the authorization process lasting two years.

Major car makers and technology firms have been working to develop autonomous vehicles, with electricity seen as the likely fuel, and some envision production within five years or so.