DETROIT/WASHINGTON (Reuters) - President Donald Trump’s administration will unveil revised self-driving guidelines within the next few months, the head of the U.S. Transportation Department said on Monday, responding to automakers’ calls for regulations that will eliminate barriers and allow autonomous vehicles on the road. “The pressure is mounting for the federal government to do something” about autonomous vehicles, U.S. Transportation Secretary Elaine Chao said on Monday in Detroit. However, Chao told reporters the federal government should be careful before setting binding rules to govern autonomous vehicles. “We don’t want rules that impede future technological advances,” Chao said. Chao was not specific about what her department’s proposals would include, or how they would differ from policy guidance proposed by the Obama administration. Companies including Alphabet Inc’s self-driving car Waymo unit, General Motors Co, Ford Motor Co, Uber Technologies Inc [UBER.UL], Tesla Inc and other are aggressively pursuing automated vehicle technologies. “We need a more concrete regulatory framework,” Ken Washington, chief technology officer of Ford, said in Detroit. Automakers could use a clear set of rules to certify on their own that an autonomous vehicle is safe, as they can now with conventional vehicles, Washington said.   Ford Executive Chairman Bill Ford Jr said at a forum in Washington he feels “quite confident” that the hardware and software will be ready by 2021 for self-driving cars. But other big issues loom. “Things like ethics,” Ford said, saying does the vehicle opt to save 10 pedestrians or the driver. Individual automakers cannot program separate ethics software for self-driving cars but must work together as an industry, Ford said.  Ron Medford, Waymo’s director of safety, said in Detroit he expected autonomous cars would appear first in “managed fleet operations,” not as vehicles sold to individuals.  Chao, in remarks prepared for delivery in Detroit, said the new rules would support industry innovation and aim to encourage “new entrants and ideas that deliver safer vehicles.” Automakers have met with Chao on several occasions in recent months and urged her to make changes to Obama-era automated vehicle rules.  Republicans on the House Energy and Commerce Committee have also been working on a package of legislation to make it easier to get self-driving cars on the road. A U.S. Senate committee is also planning a new hearing this month on self-driving cars. The Obama guidelines called on automakers to voluntarily submit details of self-driving vehicle systems to regulators in a 15-point “safety assessment” and urge states to defer to the federal government on most vehicle regulations. Automakers have raised numerous concerns about the guidance, including that it requires them to turn over significant data, could delay testing by months and lead to states making the voluntary guidelines mandatory. 