 The contribution of transport systems to potentially hazardous climate change is a significant negative externality which is difficult to evaluate quantitatively, making it difficult (but not impossible) to include in transport economics-based research and analysis.Congestion is considered a negative externality by economists.A 2016 study finds that for the United States "a 10% increase in a region's stock of highways causes a 1.7% increase in regional patenting over a five-year period."