In the American as well as the German Highway Capacity Manual, the quality of service that is determined by different measures of effectiveness can be described by six levels of service. While there are five levels (A to E) to distinguish undersaturated flow conditions, only one level of service (LOS) describes congestion. A differentiation of oversaturated traffic flow conditions (LOS F) is not possible so far. As traffic networks have become more and more congested this is insufficient, and instruments are needed to identify the worst bottlenecks. The paper presents approaches for a division of LOS F into differentiated sublevels of saturated flow (F1 to F4) and proposes corresponding calculation methods. Different traffic flow models and queuing equations are considered for the estimation of travel times as a function of traffic demand. Four potential methods for standardization are discussed that differ in the performance measures used. They are described by using freeway sections as an example, but they can be applied for all types of road facilities. The methods are analyzed and compared in terms of their significance, their sensitivity, and the availability of input data. On that basis, a 24-h analysis of traffic performance is recommended for implementation within the guidelines because it is able to evaluate congestion in a multidimensional way. The evaluation methods developed within this research can be used to assess the quality of service on existing congested facilities depending on traffic demand. They can help decision making and ensure that funds are spent efficiently.
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