The best reason for a ceiling IMO is to avoid tainting public perception. If the signboards are showing a price of $65 when it's usually $2, people might feel like they are being gouged and sour on the toll (& vote it out). Like what happened with Uber's surge pricing.
if the prices are set in advance, it hamstrings the effectiveness of the fee though. you would prevent it from working during the usage spikes where it is most needed. I think a fair solution would be to lock in the price when you get in your car. this way you can make an informed decision about whether to travel at that time, but also allow the pricing to be as responsive as possible.
