Cell phone use in vehicles is banned in Japan. Here, the National Highway Traffic Safety Administration isn't going that far. But it's issued a formal warning against drivers using cellular phones and other distracting electronics equipment.

The NHTSA says phones, navigation systems and e-mail pose a growing threat to driver safety.

Regulators estimate distracted drivers are involved in one out of four accidents today. They feel the number will escalate as automakers put more electronics into vehicles, creating virtual offices on wheels.

Rosalyn Millman, NHTSA's acting administrator, says companies must understand the risk implications of their devices, design them to minimize risk and inform consumers of the dangers they pose.

Karenann Terrell, director of e-vehicles for General Motors Corp.'s e-GM, says the automaker adheres to a philosophy of keeping the vehicle between the lines and designing systems with a minimal number of steps.

Fear of litigation has some employers considering prohibiting employees from using such devices on the road.

The Alliance of Automobile Manufacturers says it's concerned state legislators will take the ball and legislate where the federal government can't. NHTSA lacks the authority over portable phone use in cars, but its legal domain could apply to phones built into the dashboard and to navigational equipment.

Ms. Millman says technology is emerging too quickly for NHTSA to regulate, adding that NASA's first lunar capsule had less computing equipment than today's vehicles.