New research suggests trees alone aren't enough to keep roadside pollution out of adjacent environs. Hedges or a combination of hedges and trees offer a stronger buffer against vehicle pollution for roadside communities.

Scientists in Britain set up air quality monitors at several roadside test sites, each with different types of green infrastructure within a few feet of the road's edge.

Measurements showed hedges or a combination of hedges and trees offered the strongest pollution protections under variable wind conditions. Hedges alone reduced black carbon pollution 63 percent.

At test sites guarded only by trees, there was no evidence of a buffering effect at the height where polluted air is inhaled. Tailpipes release pollution only just above the ground. Tree canopies are too high to effectively block the exhaust.

Researchers shared the results of their tests in the journal Atmospheric Environment.

"Many millions of people across the world live in urban areas where the pollution levels are also the highest," Prashant Kumar, researcher at the University of Surrey, said in a news release. "The best way to tackle pollution is to control it at the source. However, reducing exposure to traffic emissions in near-road environments has a big part to play in improving health and well-being for city-dwellers."

Because the combination of trees and hedges can provide some added protective benefits in environs with specific and consistent wind conditions, researchers recommend urban planners consider weather patterns when designing green infrastructure.

"This study, which extends our previous work, provides new evidence to show the important role strategically placed roadside hedges can play in reducing pollution exposure for pedestrians, cyclists and people who live close to roads," Kumar said. "Urban planners should consider planting denser hedges, and a combination of trees with hedges, in open-road environments."

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