Dear Wim:

The applet has been modified again. Two slider were added a the right hand side to adjust the velocity.
The time trace as function of space were re-draw from the bottom, now students can find minimum of t1+t2 from the curve. (It was maximum).

About the third option: change speed to 0.5-10 m/s. Would you like it to be another simulation, or I can add another option so the scale can be changed. Let me know which way you prefer.