The problem with most rocks you find on the side of the road is that they are most likely covered in pesticides from the city/town. I would try to find out if they are in a spray zone, you should be able to call your city hall and find out that info.
Worse case if you really want them, I soaked some in a tub for several days keeping the water hot, first with bleach, then with just water, and scruber them with a scruber. I then added them to a 10g tank a few feeder gold fish in with them for about a month. after not losing any fish I assumed they were safe, rinsed them and placed them in my 55.
It is Risky, but if you take the proper precautions, and are patient, it can be worth it.