Tracking speed on rollerblades isn't something that's usually done. You could use an app on your phone to track speed using GPS, but where's the fun in that? Anurag Mishra has created a set of roller blades that has inbuilt sensors for speed, among other things.
The little device times and counts wheel revolutions to calculate the speed of the rider. He has also incorporated an ultrasonic rangefinder to work out when the wheels are on the floor (as the wheels won't give an accurate speed when off the ground). Using all this, and some simple maths, he can calculate the speed he is currently doing.
For good measure, he's also added a humidity and temperature sensors, and shows all the data on the attached alphanumeric screen. You can grab all the code etc. from his GitHub page.