Vehicular cycling advocate, John Forrester, recently passed away. The video below illustrates his ideas. In a nutshell, as a cyclist you should take yourself seriously as a road user, confidently claim the same right to the road as anybody else, and behave mostly as you would driving a motor vehicle. I have only one nit to pick: the cyclists in the video seem rather shy when it comes to claiming space, they could take the middle of the lane more often.
According to my experience, Forrester’s ideas work very well although they may take some getting used to before one can really appreciate them. Against general inclusionist trends in western societies, modern-day cycling infrastructure advocates nevertheless reject his approach, arguing that roads – or rather, segregated bike paths – should be designed for cyclists instead. In a rhetorical sleight of hand they gain approval to the truism that infrastructure design influence the safety and happiness of cyclists only to switch the general notion of infrastructure for their narrow definition later.
Dense or fast traffic can feel scary, but the real danger often looms where we least expect it. A crossroads in the middle of nowhere can be dangerous due to the angle in which roads meet. This is an infrastructure issue to be fixed by redesigning the crossroads for better visibility and perceptibility. Being advocates for a particular design, segregationists rarely discuss bicycle-friendly road design – or design objectives and tradeoffs at all.
Vehicular cycling works better on some roads than it does on others. It works where other road users do not perceive cyclists as an obstacle, either because there is ample space to pass or traffic is running so slow that passing does not really make a difference. Vehicular cycling becomes psychologically much harder for everyone when road design turns cyclists on the road into a seemingly unnecessary obstacle and therefore, a provocation. Durch designs with narrow lanes on the regular road and separate bike paths do a great job at that. Vehicular cycling would be virtually impossible here:
This road design causes the very stress bike path advocates promise to relieve through segregation. Unless you give up and comply, that is. Any honest debate of cycling infrastructure should at least acknowledge that regular roads are infrastructure and segregation is not the only viable approach to infrastructure design for cycling. If someone tries to sell you bike paths while avoiding a more comprehensive discussion of infrastructure design for cyclists, just walk ride away.
Sicher ist sicher? 
IMO, it depends on what you actually want.
If you want to cycle fast in the current traffic, Forrester’s ideas work on an individual level,at least they did for me – at least some time.
I however noted that going fast became less and less important when I grew > 40, while riding more comfortable and without stress became more and more important. And I personally found it – let’s say – extremely unpleasant to exercise vehicular cycling with three kids.
I also think, but this is rather an impression, that cycling on the street is more difficult nowadays than when I was young. Maybe because the car traffic has tripled since when I was as old as my kids are now.
On the other hand: If your goal is more political, and you want to increase the amount of cycling – e.g. because of climate change – Forrester is a dead horse to ride. Vehicular cycling just failed for decades on this topic.
Separation on the other hand historically has proven over and over again as a valuable tool to enable cycling for the masses. Historically, the people have decided. And it’s time to acknowledge this.
Personally I think, separation did so not because of being more safer than riding the road. It did so, because it created working networks for cycling. It did so by acknowledging and fulfilling the needs of cycling as a way of transport.
Politically, separation works, because it scales, while Forrester does not.
Let me clarify my own position.
I acknowledge that cyclists are a diverse bunch and have varying needs. Nevertheless I believe vehicular cycling encompasses an important set of skills and attitudes from which every cyclist can benefit if they are not to confine themselves exclusively to dedicated paths and areas for cycling. In most places it seems realistic to assume cyclists have to spend some portion of their riding on general roads along with other road users. Getting used to riding with other traffic while doing it properly can actually increase comfort and safety.
Politically I prefer to start from requirements such as: “Everyone should be able to get around on a bicycle easily and safely,” rather than favorite solutions like: “More bike lanes! More bike paths!” Designing roads and road networks is subject to constraints such as space limits and conflicting goals like cost, safety, speed, throughput, etc., and compromise is often inevitable. Sound requirements and priorities allow exploration of the entire design space and help evaluate proposed solutions, whereas a fixation on a particular type of solution only leads to haggling over wider bike lanes or bollards or whatnot.
The recent weeks with their calmed motor traffic due to the coronavirus pandemic have reminded us that segregated facilities are not the only possible way to make our roads and cities more inviting for cyclists. As you are pointing out, the density – and speed – of motor traffic has a tremendous impact on how suitable regular roads are and feel for cyclists. In addition, many design features of roads, general or dedicated, influence how pleasant or unpleasant they are to ride on a bicycle.
Therefore I consider “vehicular cycling or dedicated facilities” a false dichotomy. Politically we should not aim for either or the other, but rather for a suitable road network – and for educated, self-confident cyclists.