THANKS TO his predecessor, Brock Adams has a head start as Secretary of Transportation. One of the last official acts of William T. Coleman Jr. was publication of a remarkable study on trends and choices in transportation. The 412-page book contains a huge amount of data. And it sets out both specific questions that need early answers and broad policy issues that require major decisions.

This study brings into sharp focus the key role that transportation must play in the Carter administration - not to mention in the country's future. How people and goods are moved is central to urban, environmental and energy considerations, among others. You can't, for instance, reduce dependence on imported oil unless you reduce the consumption of gasoline. And you can't do much more to improve urban environments unless you reduce the noise, dirt and congestion that current transportation methods create.

Mr. Coleman's study poses some hard questions: Should we go on heavily subsidizing Amtrak and certain parts of the railroad system we do not now need, on the theory that they may be needed later if the oil problem isn't solved? To what extent should government use transportation policy to encourage urban density because it is more energy-efficient? How can public transit costs be allocated fairly between users and the general public and between poor and wealthy users? How far should government go in compelling consumers to buy devices for their own safety and protection? How can users of waterways and general aviation facilities be charged a fair share of what the government spends to benefit them without disrupting industries depending on them?

Until the energy crisis, almost everyone took transporation for granted. Trains ran, airplanes flew, boats went up and down rivers and somebody built highways for cars, buses and trucks. But nobody tried to fit all the pieces together and see where they were taking us. Well, we know now where that was - to a transporation system full of inefficiencies and inequities, not to mention pollution and accidents. The Carter administration has the chance to set the country on a different course.