SINCE the time of Alexis de Tocqueville, writers wanting to take the moral, political and cultural temperature of the United States have done so by going on the road, observing its inhabitants, recording their often unusual behaviour and speaking to them in inns, truck stops and diners. In The American Future, the academic and writer Simon Schama hits the road to record the US at a time when the country faces enormous challenges. The economy is teetering on the edge of a major recession, troops are committed in two foreign wars, and the country faces an electoral choice between two candidates who are both standing as the embodiment of change. Belief in the future is a traditional American virtue: how is it bearing up under the strain of events?

As an observer of the American scene Schama has many strengths; he has been engaged with the country's seismic political upheavals since he was a student in the Sixties. He lives and works in the States, and he knows and understands a lot about its history. But unlike de Tocqueville, Schama was accompanied on his travels by a film crew, and this book shows signs of having been written quickly, under the dual deadline pressure of publication and a TV edit.

It begins with hope, with the victory of Barack Obama in the Iowa primaries; the slow realisation of Hillary Clinton's supporters that they had lost the day, and the delighted excitement of team Obama that their candidate now stood every chance of winning first the nomination and then the presidency. From the votes cast in Theodore Roosevelt High, Des Moines, on January 3, 2008, Schama is off around the country, spinning observations about the role of hope, ambition and future mindedness in American public and political life. He ranges freely across the nation, and between past and present. The people he encounters are compared or contrasted with their historical precedents. Schama's pen portraits are vivid and engaging.

His journey takes him to Texas, to the South. He describes his own history, intertwining it with the journey he is taking. Readers may find themselves wondering whether there is a map for the journey he is taking, but the scenery rolls by in an engaging way all the same.

But this is more than just a retread of well-covered sources. Schama is particularly strong and interesting on the history of the Chinese in America, describing in unflinching details the "pogrom" which saw Chinese Americans in San Francisco massacred and the irony of the Asian workers who re-entered the country via the Mexican border under the noses of border guards who thought that Mexicans and Chinese looked the same.

He is attentive and respectful of the important role religion plays in American life, and resists the temptation to stereotype or send up the faithful. The future mindedness of American religion has spilled out into the rest of the culture; no wonder we routinely speak about "faith" in the future.

But there are ironies in America's current situation that Schama barely touches on: the cause of the economic collapse which threatens American business and has spurred a $700bn rescue package is, in one sense, American faith in the future. American householders believed that their houses would continue to grow in value; banks took the same gamble. Over-optimism about the future can torpedo it more effectively than pessimism ever could.

Schama's free-wheeling, free-associating style may work as a television voiceover but on the page some of his observations can seem thin, shrill and argumentative. This reader could also have done without the self-congratulatory autobiographical scenes in which Schama, the distinguished academic, is invited to dinner with Gordon Brown and the US president and ends up in conversation with George W Bush about the Texan border.

Compared with the book which accompanied Andrew Marr's magisterial History Of Britain, The American Future is provisional and sketchy. Schama has a keen eye for the ironies of American ambition and hope, but his comments often seem too fleeting and superficial to be entirely convincing. He skates over real differences and divisions in American society and is too optimistic about Obama, a candidate who, according to some polls, is on level pegging with, or even trailing, his Republican rival.

The future is coming, whether we recognise it or not. Whether we like the American future, and the way it will impact on ours, remains to be seen.