A few other characteristics of aerial photography that should be 
mentioned are: 
— a free choice of the basic areal unit (grid cell, traffic zone, 
census district, etc.); 
- permanency of record: data collection afterwards is possible by the 
separation of observation and interpretation, and the permanency, 
repeatability and consistency of the recording basis; 
- reliability: high recording capability of photographs, measurements 
can be checked easily} 
- mo influence on traffic behavior by the remote sensing aspect, 
important in the case of e.g. speed measurements; 
— economy: savings in time, manpower and cost reported in the lite- 
rature when compared to field surveys. More hard proof is needed, 
however, as the data are insufficiently documented. 
The data take-off and reduction is a bottleneck: automation or 
machine-assisted interpretation is not yet well developed. The problem 
still is the machine-identification of vehicles, which is especially 
critical when a high number of observations has to be transferred 
manually from the aerial photograph to computer-compatible input. To a 
certain extent, the problem can be overcome when traffic engineers 
revise their data specifications and data requirements are adapted to 
the economic capability of the aerial source to provide these data. 
The important limitations of aerial photography as a data source should 
be mentioned: dependency on adequate flying weather (no rain or very 
low clouds), sufficient light (nights are excluded for normal photo- 
graphy), flight restrictions (air safety). Moreover, vehicles in 
parking garages and tunnels, or otherwise hidden from aerial observa- 
tion (by radial displacement of high buildings and sometimes under 
trees or in the shadow areas) cannot be recorded. Also, superior other 
sources exist for parameters like trip production of individuals, 
trip motive, car ownership, income, etc. 
It is logical to concentrate on those characteristics where the per- 
formance of a data source is optimal. Application of aerial photogra- 
phy will, therefore, in principle be feasible when the information 
demand can be satisfied by one or both of the main characteristics of 
aerial photographs: 
— continuity in space (but sampling in time) and 
- simultaneous collection of several types of data or for several users. 
Because of the data take-off problems, the technique is performing 
better in the study of static aspects of traffic (e.g. parking 
accumulation) than of dynamic aspects (e.g. speed measurement). 
However, certain parameters, like headway and acceleration are diffi- 
cult to collect with ground methods (except cine-camera or video) and 
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