I thnk I've solved the mystery of why that turn.
Google maps has just now directed me to make the turn onto Lister Street.
It's not a route I would choose nor expected but seems to be cutting a common bottleneck and slow traffic in the tunnels.....the route it showed last night.
Even without that turn, based on my playing about, you would have carried straight on to a major island, turned left there to go towards city and entered the zone then.
To avoid the zone you would have had to have carried on straight at the major island.
Google maps does indeed seem to take you over to Spring Hill area.
I cannot find any option in google maps to avoid CAZs.... I can avoid tolls, such as London's congestion charge.
This may seem irrelevant but is actually helpful, as are the three pictures and videos showing the approaches to the turn you made. I assume the videos are included to cover the options that you would have had and to show not only the entry signs but the pre warning, blue signs.
I think Birmingham have put all three in to show the approach signs (blue) and the zone entry signs.
But seem to be a little disingenuous with the information provided.
Only one of the three warning signs shown has distance information, the two on the main road more or less saying the CAZ is to the Left/Right...not if you turn left/right you will be entering the zone.
Which the distance on the one sign does imply.
The main road is a busy road BTW, not beyond the realms for the blue signs to be blocked by a lorry or bus.
Now compare Video 1 to the other two and the photos of the junction entry sign.
You approached as per Video 1 (car 1). The video shows a brief glimpse of the side of the sign and it is not clear what the sign says or is. The video stops before the lights change and the car makes the turn, we have no chance to see how clear the sign is.
Vid 2 and 3 show the progress across the lights and that the entry sign is very clear.
The photos of the entry sign are from straight ahead, ie from the viewpoint of Vid 2 and 3, there is no photo of the sign from the viewpoint of Vid 1.
Nor does google streetview show this sign clearly as you approach or static at lights, even with the ability to scan.
The sign does become clear as you make the turn but really too late and no guarantee that a driver will see it, concentrating on the turn and whether the road is clear.
I will point to the CPZ signs on Lister Street..... they are some 50 yards from the junction and on both sides of the carriageway, a far more visible position.
Once in the zone, there are no repeater zones so realistically, if you miss that sign, you have no way of knowing that you have inadvertently entered the zone.
Putting it all together there are the bones of a defence, how an adjudicator would view it is another matter.
A stranger to the city with no knowledge of the CAZ
Signs not in Highway Code or generally known outside of CAZs
Pre warning signs not seen, possibly due to a large vehicle.
Google maps choosing to direct you to a turn in the only direction where the zone entry is not clearly visible
Birmingham seemingly aware of this lack of clarity as this direction is the only one where their car footage or photos do not show the sign. Their car footage (car 1) ending before the turn in contrast to Car 2 and 3.