Taking a bus along the BR-163 is an adventure sport. When it is dry, the ride is an exhilarating slalom between gigantic potholes. When it is wet, the bus gets stuck in the mud and the passengers are expected to pull it out by rope.
The 1,100-mile road is the main north-south artery of the Amazon rainforest. It is also the most controversial road in Brazil, built in the 1970s to open up the jungle to colonisation - forgetting, of course, that many indigenous Indians lived there already. It has become a frontier of deforestation. Now President Luiz Inacio Lula da Silva has announced that one of the major projects of his second term, at a cost of $350m, will be to pave the 600 miles of the road that is still a dirt track.
Roads bring human activity, which has always meant a plundering of natural resources. Yet Lula believes he can develop the region without increasing destruction. The stakes are high, since the area of influence of the BR-163 is a quarter of the Brazilian Amazon. 'The problem in the past is that the government has not had presence in the area,' says Muriel Saragossi, the government's co-ordinator for the Amazon region. 'We now have an integrated vision.' The 'Sustainable BR-163 Plan' involves 20 ministries and is Brazil's most ambitious attempt ever to reconcile growth and conservation.
The road stretches from Cuiaba, near the Bolivian border, to Santarem on the banks of the Amazon. On the first 450-mile paved section the rainforest has been transformed into rolling fields as far as the eye can see. The main crop is soya. Soya - half of it exported to the EU - is the economic force behind the road project. If the BR-163 is paved to Santarem, with its deep water port, farmers could export soya along it. 'This will cut the road journey to the market by 600 miles as well as a similar distance by sea,' says farmer Nelson Piccoli in Sorriso. Piccoli, like other farmers, resents the suggestion that soya is responsible for razing the Amazon: 'We did not destroy this region. We transformed this region from native vegetation to agricultural production. What you are seeing here is how we are supporting humanity. You cannot survive without eating food.'
As I travelled along the BR-163 I was surprised by how much the environmental message seemed to have got through to the timber industry. In Sinop, a lumber town, a building was emblazoned with the words Green Party. Paulo Fiuza, the local Green leader, is a former logger. 'Just because you work in the timber industry it doesn't mean you can't be an environmentalist,' he says. If they carry on destroying the way they have been, he said, they will destroy the land that has brought them wealth.
For almost two-thirds of its length the BR-163, however, is a track. Even though the road is barely passable for several months of the year, settlers came here in their tens of thousands. Here much less of the rainforest is destroyed - but the social problems are much worse. It is not just because buses get stuck in mud; it is that the region is lawless. 'We are completely abandoned here,' says small farmer Irineu Matthes, in Castelo dos Sonhos, a town of about 6,000. 'The government is not present at all. Here we are at the hands of fate.' A week after I left, two local people were assassinated.
Most murders are over land. The government encouraged settlers, but only gave a small minority title. Those who were the most violent kept the largest plots. The largest town on the unpaved section is Novo Progresso (pop. 40,000). The cattle herd here has boomed from 50,000 a decade ago to a million.
'No one put a sign at the beginning of the BR-163 when we came here saying that it was forbidden to destroy the rainforest,' argues Rancher Jose dos Santos. 'Why do we have to pay such a high price because the rest of Brazil - and certainly England - has already destroyed its forests? We just want a little space where we can live and work with diginity.'
For the lorry drivers of the BR-163, the paving will make their lives easier, but at a high cost. 'You used to see tapirs, capybaras - even jaguars - by the side of the road. Now you hardly see anything,' says lorry driver Gustavo Hering. 'When the paving comes, you'll be able to get everything out - and you will finish off the forest completely.
· Alex Bellos's film on the BR-163 will be broadcast tomorrow on Newsnight, BBC2, 10.30pm