## 3 OSM Street Network Evolution
### Topology Errors and Turn Restrictions  
A graph is generally required for a routing application that represents a street network and also comprises nodes and edges. Due to this fact, it is essential that the graph is topologically correct and that it does not contain any errors. OSM data is not routable in its standard form [41, 42]; however, within the OSM project, attempts are being made to record the street data correctly topologically, but this topology cannot be used directly for routing without additional data preparation. During this preparation, procedure junctions must be localized by searching for nodes that are used by several streets, and streets must be attributed to these nodes accordingly. However, errors do occur in the OSM dataset. We have examined the entire route network for Germany to find possible topology errors. In doing so, we identified errors in the topology similar to those visualized in Figure 12. The first possible error that can occur is that the junction cannot be determined as such, as the ways do not share a common node (1). Second, duplicate nodes or ways can cause an error (2), and third, the streets do not cross or lack information and they simply overlap (3). We converted the annual datasets (2007-2011) of OSM into routable street networks and searched for possible topology errors. The topology errors for non-linked streets were determined by measuring the distance between the two applicable streets, which should not be greater than 1 m. It can be clearly seen that the number of such errors has decreased over the years and remains high only for routes of cyclists or pedestrians (Figure 13a). The results of the second analysis for possible double streets also showed that the quality has continually improved here, at least in the street network for car navigation (Figure 13b). The number of errors for the third analysis, which shows the results of the error for intersecting streets without any shared nodes (Figure 13c), remains relatively constant, with the exception of the \"other routes\" data group. During random sampling, it happened that some of the errors that were identified were based on attribute errors in the dataset. For example, the information that the street is in fact a bridge was missing. Figure 12: OSM Topology Error Types. Turn restrictions constitute an essential component of routing applications. In a worst-case scenario, serious street accidents can occur should they be absent or incorrect. There are several different types of turn restrictions. In general, two types can be differentiated: requirements and prohibitions. Requirements prescribe the only possible way(s) to turn or travel at a junction. Prohibitions, on the  
Figure 13: **(a)** OSM Topology Errors; **(b)** OSM Duplicate Nodes or Ways Errors; **(c)** Lack of Information Errors. other hand, indicate where it is not permitted to travel. In the following preliminary comparison (Table 2), the total number of turn restrictions of TomTom and OSM for Germany are compared. The difference between TomTom and OSM totals almost 146,000. As such, TomTom currently has five times more turn restrictions available for Germany than does OSM. Although the number of turn restrictions available in the OSM dataset is continually increasing, it will probably take several more years before OSM achieves the same level as TomTom, based on the current status and development. The biggest issue during this analysis was to adjust TomTom's dataset, read the turn restrictions, and convert them in such a way that the OSM data would be applicable for a comparison. In addition to the distribution of information for turn restrictions over several attribute tables and datasets in TomTom, the existing restrictions also had to be filtered. For example \"automatically calculated\" turn restrictions or those prohibiting turning into a \"residents only\" street were among the restrictions that have been filtered out of the TomTom dataset.