In Mobbs v. Central Vermont R., 155 Vt. 210, 583 A.2d 566 (1990), the Court held that "generally, when the party seeking admission of the evidence can show substantial similarity of conditions, evidence of no prior accidents is admissible to show: (1) absence of the defect or condition alleged; (2) the lack of a causal relationship between the injury and the defect or condition charged; (3) the nonexistence of an unduly dangerous situation, or; (4) want of knowledge (or of grounds to realize) the danger.' " Id. at 226-27, 583 A.2d at 575.