Depends how you define "equal". One approach is simply to scale the charge by income -- effectively, convert time to money, and charge you a congestion tax of some amount of money-earning time. "6 minutes" is 1/10000 of your annual income -- $2 for someone earning $20,000 per year, $20 for someone earning $200,000 per year.

But does a vehicle with several people in it pay for the max, min, median, average, or the driver's time? I suspect "driver" is easiest, it seems like it might work but I'll bet there are some screwy ways to game that rule, too.