1. A computer-implemented method for calculating road segment cost for energy efficient routing, comprising: receiving probe data collected via a probe for one or more vehicles traveling along a road; selecting a road segment with associated probe data; dividing the road segment into sub-segments; calculating, using a processor, an average speed for each sub-segment using the probe data; calculating, using the processor, a speed slope for each sub-segment as a function of an average speed of the sub-segment and an average speed of at least one adjacent sub-segment; calculating, using the processor, a sum of speed slopes by adding the speed slope values for sub-segments having a negative value of speed slope; and classifying the road segment based on the sum of speed slopes.