Speed Is A Scalar:

The standard unit of speed is meter per second (m/s). A car driving all along Route 52 may have a cruise control device which you can set at, say, 25 m/s. Then, supposing that the cruise control works correctly you will be traveling, relative to the pavement, at a steady speed of 25 m/s. This will be true whether you are on a level straightaway, cresting a hill, rounding a curve, or passing the bottom of a valley. Speed can be expressed as an easy number, and the direction is not significant. Therefore speed is a scalar quantity. In this conversation, let's denote speed by the lowercase italic letter v. Speed can, obviously, change with time. When you hit the brakes to avoid a deer crossing the road, your speed will reduce suddenly. As you overtake the deer, relieved to see it bounding off into a field unharmed, you choose up speed again.

Speed can be thought as an average over time or as an instantaneous quantity. In the foregoing illustration, assume that you are moving along at 25 m/s and then see the deer, put on the brakes, slow down to a least of 10 m/s, watch the deer run away, and then speed up to 25 m/s again, all in time duration of 1 minute. Your average speed over that minute may be 17 m/s. Though, your instantaneous speed differs from instant to instant and is 17 m/s for only two instants (one as you slow down, and the other as you speed back up).