Dark Asphalt has a very low Albedo* which means that it absorbs more heat than an object with a high Albedo such as a highly reflective white roof.

What can be done to reduce the Heat Island Effect?
- Plant more canopy type trees
-Use lighter colored materials for roads and parking lots
-Use high Albedo materials for roofing

*What is Albedo?
-Reflectivity + Emissivity = Albedo
-Reflectivity: the percentage of incident light or electromagnetic radiation that is reflected.
-Emissivity: the rate at which absorbed energy is radiated from an object.

(taken from: http://www.phoenixcoatings.com/heat_islands.php)