The Alps constitute a natural barrier to transit traffic. Nonetheless, nearly 150 million people cross the Alps every year, 83% by road.
The dense road and rail networks in Alpine valleys exacts a heavy toll through space-eating traffic infrastructure, noise, and air pollution. It is also a leading cause of habitat fragmentation constituting a major barrier for Alpine species.
Additionally, so-called domestic (inner-alpine) traffic has grown exponentially, now constituting the bulk of all Alpine traffic. This domestic traffic is distributed over many different roads so that all main valleys and most side valleys are heavily polluted.
Increasing levels of nitrogen dioxide emissions is damaging forests and other ecosystems through acid rain and the introduction of damaging nutrients.
Tourism is a major driver of traffic in the Alps, especially in high remote areas with tourist resorts and scenic secondary roads. Air pollution in these areas is even more serious due to the high altitudes.