The basic idea of a woonerf is to have a street where cars, bikes, and pedestrians all share the same space in a safe way. Traffic calming techniques, signs, and the elimination of mode separation are all used to get cars to travel no more than walking speed on these residential roads. This not only allows all users to safely use the road for travel, but also allows children to play on the street without fear of harm, an important feature in row-house neighborhoods that generally lack dedicated yard space.
This "first woonerf" (apparently there is some debate whether this really is the first one) has some key features that are used to calm traffic through the neighborhood. The first is the use of special paving materials and raised areas. While the main road space is paved using the standard brick pattern found all over Delft, the intersection is raised up to sidewalk level and is paved in a slightly different pattern and color of brick. Speed humps are also used at regular intervals and are visually distinct in the same way. This treatment sends a visual and physical message to drivers that they are entering a pedestrian zone and must be cautious. A similar treatment is often used in the Netherlands where local roads intersect with major arterials--the sidewalk continues at grade through the intersection, which cars are forced to go up and over in order to access the road. This tells drivers that they are guests in that space and in my observations they did in fact drive very carefully in these situations.
Another interesting treatment found in this woonerf is known as a "chicane." This refers to any measure that forces traffic to move laterally rather than going straight. In this case, large stone bollards mark the transition point and parking is moved from one side of the street to the other. Drivers naturally slow down whenever they do not have a clear straightaway to drive down, especially in such a tight roadway. I suspect that chicanes play a much stronger traffic calming function then speed humps for this reason. In Portland, it is common to see cars driving very fast over the speed humps on our neighborhood greenways. The suspension on most cars can deal just fine with the wide, shallow speed humps used on these roads, and many drivers decide they can deal with the discomfort of going over them at high speed. With a chicane, by contrast, it is unlikely most drivers would be willing to take the risk of switching from one side of the road to the other at high speed.
I was surprised to find that this first woonerf still has a separate (albeit tiny) sidewalk, as my prior understanding of a woonerf was that it eliminated separate facilities for different modes. In later tours, in fact, we did encounter some newer woonerfs in which curbs were completely eliminated in favor of a completely shared space. As I understand it now, this is a later evolution that came out of concerns that traffic-calming and signage were not really enough to create an idea of shared space.
The idea of having calm streets that are safe enough for kids to play on has been one driver of the widespread use of cul-de-sacs in suburban neighborhoods in the US, but the lack of pedestrian and bike connectivity and the use of very wide streets often prevents them from being actively used by anyone not in a car. Many planners promote the Dutch woonerf model as an alternative, both for new suburban neighborhoods and urban infill projects, but actual implementation continues to be rare due to safety concerns. US traffic engineering has long focused on signs and signals as the keys to safety, whereas in the Netherlands they seem to focus more on roadway design. There are certainly some signs, like the one denoting shared space found on many woonerfs, but these are less important than designing the roadway to discourage fast driving through use of paving materials, speed humps, and chicanes, among many other techniques.
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