In urban areas with high cycling levels and low traffic speeds like Oxford, central London, Cambridge, it's reasonable to expect driver awareness to be great enough to avoid conflict and impact speeds modest enough to avoid serious injury. Likewise country lanes, at least those that haven't become commuter rat runs, are generally safe because of low vehicle volumes and the opportunity to avoid/evade cars.
The problem for cyclists is A roads and busier B roads where there is no likelihood of lowering speed limits due to the necessity to keep traffic moving. If we accept most riders avoid dual carriageway drag strips wherever possible (which has been my experience) that leaves the majority out of town roads as at least potentially dangerous.
Some of that danger is ameliorated by good road craft but the onus is entirely on drivers to behave well and without a change in culpability laws and increased penalties for transgressors, there's no general impetus to do so. Basically, mixing it with cars travelling at impact speeds well above the 20 mph mortality line and few penalties for getting it wrong, will yield a regular supply of serious 'accidents' because driving standards, motor vehicle numbers and relatively low numbers of cyclists make it inevitable.
Even if one believes the absolute number of deaths and serious injuries are few enough statistically to make regular main road cycling viable, traffic volumes and driver behaviour still make such roads unpleasant enough to be a barrier to cycling take up.