I would use Spatial Join prior to the Hot Spot analysis. Use spatial join to calculate average speed of cars on segments of your road network. In order to do that set Target feature as your road segment, join feature as your car points. You will see list of all fields below, select the field that consists of your car's speed. (Since not all points will intersect with your segment, you can set a tolerance for the join) Right click on it and select "Mean" from calculation method.
When it finishes calculation, each of your segments will have average speed of cars that passes through that segment. After you have this information you can use any statistical method to define hot and cold areas. I would basically use reclassification as we already know what is low and what is high. That's up to your requirement.