These obstacles force people to pay attention and therefore increase road safety. And roads are not for cars only, so at least in town it's prudent to ensure people drive slowly.
If someone is not paying attention, all this trickery will simply make his/her car drve awkwardly, or in the wrong lane, or going as fast as if the obstacle wasn't there (thus reducing margin for error and increasing risk of an accident). On the contrary, if someone IS paying attention he/she may slow down out of fear (crashing, puncturing a tire, fear of being unable to make the car fit) and make circulation more difficult. Moreover, if we talk about bends or other, I have seen bends which have actually been badly designed on purpose (the main purpose being making people slow down and the actual result being getting cars to face blocks of concrete popping out of the blue exactly in the bend line. This happens at legal speed too, and bad weather or night don't help very much either).
I must add that systems like narrowing the streets reduce the number of car that can pass through a certain place in a fixed amount of time ,and narrowing the streets reduce the possibility for a vehicle to overtake safely, or just legally, another vehicle, even cars overtaking bycicles. Yes, traffic average speed will be reduced, but it will also be more changeable (traffic will react even at small modifications in vehicle speed and number) and unpredictable, and traffic will be prone to complete blocks in case of crashes or mechanical failure or pure and simple heavy traffic.
We must also remember, at this point, that reducing average speed under a certain limit and, most of all, inducing this by making vehicles ride at non-constant speeds by accelerating and braking constantly is a good way to raise both the mechanical wearing and the fuel consumption, two things that increase pollution. Plus, it is a raise which happens exactly in the most densely populated, and residential, areas, precisely where every government is trying to reduce this sort of things.
Last but not least, reducing traffic speed on roads where surface public transportation also run means actively reducing the average speed of buses and trams and the like.
So, ok, average speed is reduced, which is safer for certain cathegories of people (and the less protected, I must admit) but at the cost of heavier and less fluid traffic, higher risk for accident to motorists (so less safety for them), higher costs (for both vehicles and road maintenance) and higher pollution.
I still maintain that education (for both motorists and pedestrian/cyclists), better road system design (make transfer roads that avoid residential areas and make residential pedestrian areas surrounded by roads and parkings), better public transportation, and active patroling are far more effective solutions to reduce risks while keeping faster roads and low pollution.
But I know that putting blocks of concrete is less expensive and requires less efforts.
Too bad this least difficult way of letting people driving around without knowing what they are doing and desinging senseless streets is getting us to a constant lowering of speed limits and growing of prohibitions and limitations and frustrating absurdities. (And yes, I predict that even our belovedgerman autobahns will get a low speed limit, sooner or later, out of "safety" and "pollution" issues).
