SRB separation occurs at an altitude of about 30 miles The separated boosters then coast up to an altitude of 47 miles and free-fall into an impact zone in the ocean about 158 miles downrange. The so-called splash "footprint" is in an area about 7 miles wide and about 10 miles long.
In the case of Baikonur, the Altai region had the bad luck of being downrange and thus it was peppered with spent Proton stages and toxic storables. Russianspaceweb has a very detailed page about downrange Baikonur, spent stages, and drop zones. http://www.russianspaceweb.com/baikonur_downrange.html