I used to be a Traffic Police officer covering the Blackwall & Rotherhithe tunnels. When there was an incident, such as a crash, we had to work down there. Let me tell you that the atmosphere is awful when there is stationary traffic and we were allowed to spend only 20 minutes in there at a time. You emerge with major headaches and an almost coal-black face - most unpleasant!
BTW, yes the tunnels are curved so that horses do not see daylight until the last possible moment. The Blackwall Tunnel is Europe's busiest road tunnel and when built in 1896 it was gas-lit. It had two-way traffic in there until 1965, and I would love it if the engineers who designed it could see how it is used today - wouldn't they be amazed?