However, I did see one proposal to add a surcharge to cab fares within a certain congestion zone, relating to a proposal for personal vehicle congestion fees. (Source – search for “higher taxi fares” on the page)

That got me thinking that perhaps instead of doubling or halving the fare during peak or non-peak times respectively, the flag drop should be adjusted. In San Francisco the flag drop, the price at which the meter starts, is $3.10. Perhaps at non-peak times it should start around $1.00 while at peak times it should start around $6.00.

It might be a more simple way of implementing variable pricing.

Furthermore, the City could make it widely known during which hours each flag drop was valid. Say between 3 am and 5 am everyday flag drop would be $1.00, while between 5 pm and 12 pm Thu, Fri and Sat flag drop would be $6.00. All other times it would remain $3.10.

But, again, I think variable supply (peak medallions) would be more readily implemented and more widely supported by both residents and drivers than variable pricing.