To measure a distance, click one the line measure symbol (1).
Click on the start point and then on the end point of the distance, you want to measure. Always have a look at the tool tip, if you want to know, what to do next.
If necessary, you can adjust the position of the points of an existing line.
Press the <Shift> key while drawing a line to make the line parallel to one of the axis.
For lines parallel to the x axis, the unit s (3) is used and for lines parallel to the y axis, the unit km/h (2) is used.