Millions of people who live near a motorway are at greater risk of dementia, warns new research.
The study shows dementia is more common in people who live within 50 metres (54.6 yards) of a motorway than those who live further away
But researchers found no link between traffic exposure and Parkinson's disease or multiple sclerosis.
The observational study, involving 6.6 million people and published in The Lancet, estimated that up to one in nine cases of dementia (seven to 11 per cent) could be attributed to living near a major road, and that the link is strongest for those living closest to heavy traffic.
For the purposes of the study, a major road was classed as a morotway in Britain such as the M1 or M4, major state or interstate highways in the United States, or the 400-series highways in Ontario, Canada.
Previous research has suggested that air pollution and traffic noise may contribute to neurodegeneration, with one study finding that living near a road was associated with reduced white matter and lower cognition.
But the paper in The Lancet is the first to investigate the link between living close to heavy traffic and the onset of major neurodegenerative diseases.
The researchers tracked all adults aged between 20 and 85 living in Ontario, Canada - around 6.6 million people - for over a decade from 2001 to 2012.
They used postcodes to determine how close people lived to a road and analysed medical records to see if they went on to develop dementia, Parkinson's disease or multiple sclerosis.
Almost all people in the study (95 per cent) lived within one kilometre of a major road and half lived within 200 metres of one.
Over the study period, more than 243,000 people developed dementia, 31,500 people developed Parkinson's disease and 9,250 people developed MS.
While there was no association between living near a road and Parkinson's or multiple sclerosis, dementia was more common the closer people lived to busy roads.
The risk of developing dementia reduced as people lived further away from a main road - with a seven per cent higher risk in developing dementia among those living within 50 metres, a four per cent higher risk at 50 to 100 metres, a two per cent higher risk at 101 to 200 metres and no increase in risk in those living more than 200 metres away.
The researchers also found that long-term exposure to two common pollutants -nitrogen dioxide and fine particulate matter - was associated with dementia but didn't account for the full effect, meaning other factors are also likely to be involved.
These could include other air pollutants or noise from traffic, according to the researchers.
Lead author Doctor Hong Chen, of Public Health Ontario, said: "Despite the growing impact of these diseases, little is known about their causes and prevention.
"Our study suggests that busy roads could be a source of environmental stressors that could give rise to the onset of dementia.
"Increasing population growth and urbanisation has placed many people close to heavy traffic, and with widespread exposure to traffic and growing rates of dementia, even a modest effect from near-road exposure could pose a large public health burden.
"More research to understand this link is needed, particularly into the effects of different aspects of traffic, such as air pollutants and noise."
The study estimated air pollution exposure based on postcode, so didn't account for each person's exposure.
The researchers said that because the study was observational it cannot establish causality, but it was designed to control for socio-economic status, education levels, body mass index (BMI) and smoking meaning the link is unlikely to be explained by these factors.
Dr Lilian Calderón-Garcidueñas, of Montana University, said of the findings: "The health repercussions of living close to heavy traffic vary considerably among exposed populations, given that traffic includes exposures to complex mixtures of environmental insults.
"We must implement preventive measures now, rather than take reactive actions decades from now."