You may find that not one size B double fits all as no country is the same .
Having had the experience of specifying many different trucks in various continents to suit local governments you may find your self well placed by not being greedy on length initially, but generous on safety features and procedures to mitigate introduction risk.
When the late Anders Lundstrom and myself lobbied the government in Malaysia for the B double introduction we struck the same obstacle i.e length on safety , initially authorities only allowed a 18 meter b double in 2009 which grew up in length to 24 meters today.
Various independent studies then showed that for productivity, safety, bridge protection and environmental reasons greater length was the way forward.
What really struck was that the nimble 6x2 B double was much more flexible and productive than it's 4 axle quad cousin and added up to 20 % increase in productivity with a superior turning circle in tight traffic situations.
Introduction of B double Driver training, brake testing, setting speed limiters, EBS, ESP Stability Control, can routers, Disk Brakes, retarders and compliance to i.e UN-ECE , PBS standards will all help and should be mandatory to have a safe combination outcome regardless of length. Once the safety is proven the additional vehicle length will come over time. Here in NZ we are still working to get a simple 0.5 meters from 23 M to 23.5 meters for logging trucks. The Malaysian B double concept has proven safe to date like it's Australian and NZ cousins with millions of km's under it's belt in dense traffic despite all harsh critics at the time. Some photos attached of the initial test prototype .
With Kind Regards