The U.S. transportation system was built for the typical weather and climate experienced locally. Moderate changes in the mean climate have little impact on transportation. However, changes in weather and climate extremes can have considerable impact on transportation. Transportation relevant measures of extremes have been changing over the past several decades and are projected to continue to change in the future. Some of the changes are likely to have a positive impact on transportation and some negative.

As the climate warms, cold temperature extremes are projected to continue to decrease. Milder winter conditions would likely improve the safety record for rail, air and ships. Warm extremes, on the other hand, are projected to increase. This change would likely increase the number of roadbed and railroad track bucklings and adversely impact maintenance work. As the cold season decreases and the warm season increases, northern transportation dependent upon ice roads and permanently frozen soil would be adversely affected while the projected commercial opening of the Northwest Passage would result in clear benefits to marine transportation.

The warming would also produce a side benefit of shifting more of the precipitation from snow to rain. But not all precipitation changes are likely to be beneficial. Heavy precipitation events are projected to increase, which can cause local flooding. At the same time, summer drying in the interior of the continent is likely to contribute to low water levels in inland waterways. Strong storms, including hurricanes, are projected to increase. Coastal transportation infrastructure is vulnerable to the combined effects of storm surge and global sea-level rise.

Transportation planning operates on several different time scales. Road planners typically look out 25 years. Railroad planners consider 50 years. And bridges and underpasses are generally designed with 100 years in mind. In all cases, planning that takes likely changes into consideration will be important.