The real (modern) london bridge is mostly bus lane and pedestrian traffic. 2x bus lanes, 2x wide pavements, and 2x car lanes (probably mostly taxis). I'd guess - from when i used to regularly use it 10-15 years ago, the pavements move the most people by a decent margin , especially at rush hour. I always thought they should close one of the bus lanes at rush hour to give to pedestrians.

It's not clear which bridge they're trying to draw though, normally people depict tower bridge which is way narrower. I think just 1 mixed lane in each direction - that is often a log jam of buses and cars - still fairly decent width pavements though, but no where near the rush hour (pedestrian) traffic of London bridge. Thats just because its a worse location.

Of course the nursery rhyme refers to the old medieval london bridge that was covered with houses and shops and stuff. Of corse in those days people went to Southwark for different reasons.

Maybe they're depicting the dartford crossing?