They just want to test your brakes. If you can do 40mph to 30mph in one block then maybe they will make it half a block and see if you can still cope up. If you still can then they will shortened the distance to maybe 20 meters or until you get caught.
MJ is a good product, an excellent invention that will eventually pave way for other great inventions but at $20 a year, please don't rely for it as your all in one telecommunication tool. I mean get a back up or make MJ your back up. A $20 back up phone is perfect especially for travelers.
I do have my main VoIP service from Onesuite but I also have MJ. Everyone needs a back up specially when it comes to VoIP.
I also use Google Voice and its useful from time to time.
Bottom line, don't expect the earth and sky for $20/year.