In Preparing and Presenting Expert Testimony, traffic engineer and accident reconstruction expert witness Lawrence Levine writes:
How then can a Traffic Engineer best assist an attorney in either developing the basis for a suit to be filed by a plaintiff attorney or defending a case, which would be the province of government agency attorneys or outside defense counsel. Note that although the two groups of lawyers often overlap by handling both defense and plaintiff work, lawyers generally focus on one or the other. As such, each attorney has a different bias and therefore a different agenda.
Attorneys are ‘advocates.’ They are supposed to take sides, no matter how they might feel personally, to provide their client with the best representation. This is not the case for the Traffic Engineer who the lawyer chooses to use either as a fact witness or as his expert witness. The Traffic Engineer in either case is expected to be totally objective, have no bias, and state opinions they have developed “within a reasonable degree of engineering certainty.” Plaintiff lawyers are paid on a contingency, if they win. Experts are paid regardless.