2 - Line as Direction

Flat, horizontal lines looke lifeless, like the floor around you. Vertical lines, like the walls seem to have a bit more life, but let's face it, their dull. A diagonal line though, that is where you get to have a little fun and can imply motion in a flat design. Line as direction is a well known tool for adding a bit of excitement in your sketch or drawing

Notice how the main characters in this other inking by Mark Brooks shows the motion of the moving cars and the scooter in the front. Also, the lines under the scooter show movement and action. Even the bridge is at a slant to further promote the chaos of this bridge in New York City.