I must have missed that post on SABRE - and what its suggesting is something so laughably outrageous (given we're talking about an off-line motorway/expressway unlikely to have many junctions not having a junction with another expressway, despite it being the only major road it crosses), there would have been kick back. Wait, found it - it was in the tinfoil conspiracy strand of that thread.I guess there’s always the possibility that a future new A1 will cross the east/west route without a junction. I think that’s being hypothesised on the “Sabre” roads forum...
Having a three-level roundabout at Black Cat, rather than other options that were perhaps more strategic in where the free-flow goes (ie Bedford-North as well as Bedford-Cambridge), might suggest that they hope that the Tempsford/Sandy development will be large, and the traffic at Black Cat would need something to help stop jams from the south. Which is relevant to E-W Rail, as it suggests that Tempsford might be a worthwhile station to build and serve, suggesting certain options over others.