What is the collateral source rule in motor vehicle accident law?

3 In Helfend this court defined the collateral source rule as 'the doctrine that if an injured party receives some compensation for his injuries from a source wholly independent of the tortfeasor, such payment should not be deducted from the damages which the plaintiff would otherwise collect from the tortfeasor.' (Helfend v. Southern Cal. Rapid Transit Dist., supra, 2 Cal.3d at p. 6, 84 Cal.Rptr. at p. 175, 465 P.2d at p. 63.)