  Urbanization leads to an increase of traffic in cities. The Macroscopic
Fundamental Diagram (MFD) suggests to describe urban traffic at a zonal level,
in order to measure and control traffic. However, for a proper estimation, all
data needs to be available. The main question discussed in this paper is: How
to derive a network-wide traffic state estimate? We follow up on literature
suggesting to base the operational estimate on the speed of limited sample cars
sharing floating car data (FCD). We propose an initial step by constructing an
MFD based on FCD, which is then used in step 2, the operational traffic state
estimation. For operational traffic state estimation, i.e., the real-time
traffic state estimation, the penetration rate is unknown. For both steps, we
assess the impact of errors in the estimation. In light of the errors, we also
formulate an indicator which shows when the method would yield insensible
results, for instance in case of an incident. The method has been tested using
microsimulation. A 26% error in the estimated average density is found for a
FCD penetration rate of 1%; increasing the penetration rate to 30\% reduces the
error in estimated average density to 7%.
