During the WTO event of 2005 the cost differential between the tunnels was closed by subsidising the privately managed tunnels. There was a significant change in volumes of traffic. If the CHT was raised to $40 then the WHT would probably have to drop their prices by $10 as they would be making too much money.
Interestingly the recent proposals from the TD on what to do with tunnel pricing looked to increase overall traffic which seemed a little strange.
I drive regularly in London, but not regularly in Hong Kong. I noticed that on a lot of weekday evenings and weekends, HK Island roads seem relatively empty (e.g. Kennedy Town, Central, Sheung Wan, Causeway Bay) compared to Central London. Central London on a weekend would be relatively gridlocked (especially around Harrods in Knightsbridge, Piccadilly Circus etc.).
Hence, I may have anecdotally gotten the impression that HK roads look relatively empty at the weekend