h[x_] := If[Abs[x] < 1, 1, -1];
f[{{a_, b_}, {v_, w_}}] := {{a + h[a] v, 
    b + h[b] w}, {h[a] v, (-0.0001) + 0.96 w + 0.0005}};
DynamicModule[{c = {}}, r := 0.04*(Random[] - 1/2);
 EventHandler[
  Graphics[{{RGBColor[0, 0, 1], FontSize -> 15, 
     Text[
      "CLICK TO SEE EFFECT OF MULTIPLE CARS ON ATMOSPHERE", {0, 
       0.8}]}, {Opacity[0.1], RGBColor[0.1, 0.1, 0.1], PointSize[0.3], 
     Point[Dynamic[c = Map[f, c]; Map[First, c]]]}}, 
   PlotRange -> {{-1, 1}, {-1, 1}}, Background -> RGBColor[0.9, 0.9, 1],
ImageSize -> 500], 
  "MouseDown" :> (AppendTo[c, {MousePosition["Graphics"], {r, r}}])]]