Environmental impact of roads

The environmental impact of roads (both positive and negative) include the local effects of highways (public roads) such as on noise, water pollution, habitat destruction/disturbance and local air quality; and the wider effects which may include climate change from vehicle emissions. The design, construction and management of roads, parking and other related facilities as well as the design and regulation of vehicles can change the impacts to varying degrees.

Air quality
Roads can have both negative and positive effects on air quality.

Negative impacts
Air pollution from fossil (and some biofuel) powered vehicles can occur wherever vehicles are used and are of particular concern in congested city street conditions and other low speed circumstances. Emissions include particulate emissions from diesel engines,, volatile organic compounds, Carbon monoxide and various other hazardous air pollutants including benzene. Concentrations of air pollutants and adverse respiratory health effects are greater near the road than at some distance away from the road. Road dust kicked up by vehicles may trigger allergic reactions. Carbon dioxide is non-toxic to humans but is a major greenhouse gas and motor vehicle emissions are an important contributor to the growth of concentrations in the atmosphere and therefore to global warming.

Positive impacts
The construction of new roads which divert traffic from built-up areas can deliver improved air quality to the areas relieved of a significant amount of traffic. The Environmental and Social Impact Assessment Study carried out for the development of the Tirana Outer Ring Road estimated that it would result in improved air quality in Tirana city center.

A new section of road being built near Hindhead, UK, to replace a four-mile section of the A3 road, and which includes the new Hindhead Tunnel, is expected by the government to deliver huge environmental benefits to the area including the removal of daily congestion, the elimination of air pollution in Hindhead caused by the congestion, and the removal of an existing road which crosses the environmentally sensitive Devil's Punchbowl area of outstanding natural beauty. (UTC)

Noise
Motor vehicle traffic on roads will generate noise.

Negative impacts
Road noise can be a nuisance if it impinges on population centres, especially for roads at higher operating speeds, near intersections and on uphill sections. Noise health effects can be expected in such locations from road systems used by large numbers of motor vehicles. Noise mitigation strategies exist to reduce sound levels at nearby sensitive receptors. The idea that road design could be influenced by acoustical engineering considerations first arose about 1973.

Speed bumps, which are usually deployed in built-up areas, can increase noise pollution. Especially if large vehicles use the road and particularly at night.

Positive impacts
New roads can divert traffic away from population centres thus relieving the noise pollution. A new road scheme planned in Shropshire, UK promises to reduce traffic noise in Shrewsbury town centre.

Water pollution
Urban runoff from roads and other impervious surfaces is a major source of water pollution. Rainwater and snowmelt running off of roads tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants. Road runoff is a major source of, , , , and polycyclic aromatic hydrocarbons (PAHs), which are created as combustion byproducts of gasoline and other fossil fuels.

De-icing chemicals and sand can run off into roadsides, contaminate groundwater and pollute surface waters. Road salts (primarily chlorides of, or ) can be toxic to sensitive plants and animals. Sand can alter stream bed environments, causing stress for the plants and animals that live there.

Habitat fragmentation
Roads can act as barriers or filters to animal movement and lead to habitat fragmentation. Many species will not cross the open space created by a road due to the threat of predation and roads also cause increased animal mortality from traffic. This barrier effect can prevent species from migrating and recolonising areas where the species has gone locally extinct as well as restricting access to seasonally available or widely scattered resources.

Habitat fragmentation may also divide large continuous populations into smaller more isolated populations. These smaller populations are more vulnerable to genetic drift, inbreeding depression and an increased risk of population decline and extinction.

Another negative effect is the amount of space roads take up. When cutting through forests, they prevent the growth of many trees as trees can not grow through paved roads. On unpaved roads, vehicle tires and foot traffic compact soil and prevent plant growth. As trees take up 2 and as they also house animals, this increases the environmental damage inflicted.

Facilitation of poaching of flora and fauna
Roads that run through forests that house edible animals may encourage or facilitate poaching. Especially in poor areas, the construction of roads has promoted not only poaching for personal consumption but also for sale (for consumption or as a pet) to third parties.

Similarly, the construction of roads in forested areas has also promoted illegal logging as it becomes easier for illegal loggers to transport the wood.

Recycling
Materials removed from a road can be reused in construction within the same project or in other locations. Road construction can also use waste materials from other industries.

Asphalt pavement is one of the most recycled materials in the United States. It is estimated that over 80% of asphalt pavement removed from roadways is reused as construction aggregate. It can be mixed into new pavement or used as a subbase or fill material. Similarly, concrete from road or building demolition can be an excellent source of aggregate.

Common examples of waste products used in road building include coal fly ash (used to make concrete stronger), asphalt shingles and shredded tires (used in asphalt pavement), ground glass and steel mill slag (used as aggregates).

In 2012, US asphalt plants used an estimated 68.3 million tons of reclaimed asphalt pavement, 1.86 million tons of recycled asphalt shingles, and over 1 million tons of other recycled materials to produce new asphalt.