Actually, if we all want to get technical here, there are two theories to answer the speed of darkness question.

Darkness is the absence of light. Theoretically it is not a physical thing like light, so the laws and measurements of physical things, like kph or mph, do not apply. Light is technically made up of photons. The less photons there are, the less color there is. If there are no photons, then there are no colors, and darkness just remains. Photons move, darkness doesn't. So technically, darkness isn't a matter that can be measured. Take what my teacher in my physics class used as an example. A table is darkness. A table cloth is light. As a table cloth is placed on a table, it covers the table (thus being darkness). If you remove the table cloth (color) all that remains is the table (darkness). The table cloth (color) can move. The table (darkness) doesn't.

Another theory is what ghettoshen suggested. Because of Newtons third law, which states that everything has an equal and opposite reaction, that would mean that light and dark are essentially equals, and thus have the same speed.

Of course, these are all just theories, and no one has come alone to prove them. Now, who says American high school students aren't smart? =P