I think you should check in with your DOT so you don't get arrested. After that, if you need instructions on how to dig, email me and I can send you something.

Definitely ASK, or risk getting arrested. I'm sure someone in your area would be willing to let you take some--consider posting it on Craigslist or such. But I DO NOT advise moving bamboo in this heat. We're seeing one day after another in the upper 90's without rain in my area--the bamboo will be dead before you lift it out of the ground in that kind of heat. I generally lift and propagate the bamboo in March (on a nice misty, showery day, but I bet it can be done in the Fall as well.--In the meantime, try to stay out of jail.:)

Although it's not the best time to dig bamboo, it's possible. I did it a couple of weekends ago in the 90F heat of St. Louis. There are several key steps though:

1) if the grove is in sun, dig as early in the morning as possible so the plant is hydrated. If it's in shade it's not as important, but morning digs are best.

2) remove some portion of the culms or branches to reduce leaf surface area

3) keep the rootballs moist and the plants in shade

4) if you have to drive some distance (especially at highway speeds), wrap the plants in a tarp or plastic so they lose as little moisture as possible

5) pot the divisions immediately (or ASAP) and keep well-watered (but not waterlogged)

6) keep the plants in full shade for a week or so. If they have significant leaf curl, remove more branches

7) move to partial sun for a couple of weeks.

I'm not saying this guarantees success, but it has worked for me on two separate occasions this summer already (on Phyllostachys aureosulcata).

When selecting, find a large bushy culm. Cut it down about 1/4 to 1/3 its height. This will reduce the water demand for the plant. Then when digging the roots, get the largest root ball as possible.
I agree with everything stated above, there is a possibility that law will stop and question you but I do not think they will arrest you. Im sure they have better things to do then waste their time with a guy digging grass off the interstate. The worst that will happen is a ticket for stopping your vehicle on the interstate. Can you park your car on a road that crosses the interstate?

If you do this now, I agree, its very hot. Keep it in a pot, soil very moist and keep the plant in the shade till you see it growing again or till fall and plant it in the ground in October-March, the earlier, the better.