Plaintiff sued Defendant for personal injuries suffered in a train-automobile collision. Plaintiff called an eyewitness, who testified that the train was going 20 miles per hour. Defendant then offers the testimony of an experienced police accident investigator that, based on his training and experience and on his examination of the physical evidence, it is his opinion that the train was going between 5 and 10 miles per hour. Testimony by the investigator is 
improper, because there cannot be both lay and expert opinion on the same issue.  improper, because the investigator is unable to establish the speed with a sufficient degree of scientific certainty.  proper, because a police accident investigator has sufficient expertise to express an opinion on speed.  proper, because Plaintiff first introduced opinion evidence as to speed. 