Calculation Rules
Public Holidays
All national (Dutch) public holidays (New Year's Day, Easter Monday, King's Day, Ascension Day, Pentecost Monday, Christmas Day, and Boxing Day) as well as Liberation Day (May 5), Good Friday, and New Year's Eve (December 31) are considered public holidays. The table indicates when the data for these days are included.
| Environment | Handling of Public Holidays |
|---|---|
| Dexter - Explorer | All days within the selected period are included, including public holidays |
| Dexter – Export | Public holidays are excluded from results if the checkbox to exclude public holidays is selected |
| Dexter – Reporting | Public holidays are excluded from results if the checkbox to exclude public holidays is selected or in standard workday products |
Vacation Days
In the export, you can specify not to include vacation days for a selected region. These are the advisory dates for primary education issued by the Ministry of Education, Culture, and Science. The excluded days can be viewed in the Excel file << request name >>_exclusions.
Weekly Aggregate
A weekly aggregate is always calculated from Monday to Sunday, excluding the first and last week in the selected period. The first week starts with the start date of the selected period and ends on the first Sunday within the period. The last week starts on the last Monday in the period and ends on the end date of the period.
When a larger aggregate is chosen than the selected period (e.g., a day while only 4 hours are selected), the average over the entire period is given, not for the entire day.
Intensities and Speeds
Vehicle Categories
The measurement locations classify traffic into vehicle categories based on vehicle length. The table below shows the various vehicle categories and the corresponding NDW indexes. For individual vehicle passages, the length classification can be selected. It may happen that vehicles are "measured" but cannot be categorized, as the length cannot be determined (e.g., skewed vehicles at detection loops). In such cases, the vehicles are counted in the anyVehicle category but not in a specific category. Vehicles shorter than 1.85 meters are either considered as incorrectly measured or as the smallest category. Vehicles longer than 25.25 meters are either categorized in the largest category or considered as two vehicles, which is also equal to the legal maximum vehicle length in the Netherlands.
| Classification with 3 Categories | Classification with 5 Categories |
|---|---|
| 1.85 m ≤ L ≤ 5.60 m | 1.85 m ≤ L ≤ 2.40 m |
| 5.60 m ≤ L ≤ 12.20 m | 2.40 m < L ≤ 5.60 m |
| 12.20 m < L ≤ 18.00 m | 5.60 m < L ≤ 11.50 m |
| 11.50 m < L ≤ 12.20 m | |
| 12.20 m < L ≤ 25.25 m |
When 3 vehicle categories are requested, and a location delivers 5 categories, categories 1 and 2 can be combined for the first category, and categories 3 and 4 for the second category.
Harmonic Mean Speed
To calculate an average value, an arithmetic mean is often used. For instance, if scores of 6 and 8 are achieved, the arithmetic average is 7. In speed calculations, the harmonic mean is often used. The harmonic mean takes into account the overrepresentation of high speeds in the arithmetic average. For example, if two athletes are measured over 1 km, with one running at 20 km/h for an hour and the other at 16 km/h, the arithmetic mean would be 18.22 km/h. However, the harmonic mean, which accounts for the overestimation of high speeds, results in 18 km/h. The harmonic mean is the inverse of the average of the inverses of the speeds and is calculated as follows:
Where:
\(v_{harm}\): The harmonic mean speed
\(N\): The number of minutes averaged
\(v_i\): The speed in minute \(i\).
Trajectory Method Travel Times
The trajectory method is a calculation method to account for the fact that you are at different points in a long trajectory at different times. This effect becomes more significant for longer trajectories. For example, if we consider a route from Arnhem to Rotterdam, the travel time at 8:00 AM can be calculated by summing the travel times of various segments at 8 AM, but someone who departs at 8:00 AM will experience a different travel time. If you leave Arnhem at 8:00 AM and arrive at Ede at 8:20, then the travel time from Ede must be considered at 8:20, and from Utrecht at 8:45, and so on. The trajectory method accounts for the fact that a traveler is at different times on different parts of the route. It is necessary for the trajectory to be divided into connected sub-segments (referred to as measurement sections), and the travel times at different times for these sections must be known (For Floating Car Data, this condition is easily met, as travel times are known at the segment level).
The trajectory method (Piecewise Constant Traveltime Based method) works according to the following process:
For each measurement minute in the request period, an exit time for the first travel time section is determined by adding the travel time to the periodStart. This exit time is then used as the entry time for the second section, and the exit time for the second section is calculated by adding the travel time for the entry minute of the second section. This process continues for each subsequent section. The travel time for the first minute of the period is then the total travel time.
Suppose a trajectory consists of 10 consecutive sections, with the first 9 sections having a travel time of 60 seconds. The travel time for the tenth section used in the calculation of the total travel time is then the travel time 9 minutes after the travel time for the first section. It may also occur that the travel time for a trajectory in the last minute of a period is largely determined by travel times outside the requested period (i.e., from later sections of the trajectory). If the travel time for any of the sections is missing, the travel time for the entire trajectory is considered missing.
Individual Vehicle Passages
For individual vehicle passages, the speed and length are stored per passage. This provides flexibility in calculating aggregates. In the export, it is possible to define your own vehicle length and speed categories.
Intensities
When exporting intensities, it is possible to select both vehicle length and speed categories, unlike speed aggregates, which only allow the selection of vehicle length categories. Since the passages are stored individually, the individual passages are summed to reach the correct aggregate. To calculate an average over multiple days, the passages are first summed over the desired period per day, then the average is determined over multiple days.
Speeds
For individual vehicle passages, there is an option to use an arithmetic average or a harmonic average. For more information about the harmonic average, refer to the section above on speeds and intensities.
Speed Percentiles
For traffic safety applications, speed percentiles are also considered. A commonly used value is the V85, the 85th percentile of speed. This is used for traffic safety analyses. In statistics, a percentile is one of the 99 points that divides an ordered dataset into 100 equal parts. The k-th percentile separates the k% smallest values from the (100-k)% larger ones. The 85th percentile, for example, is the value below which 85% of the data falls and above which 15% of the data lies. In the Dexter export module, the speed percentile is a customizable value.
Speed Violations and Offenders
The last aggregate type for speeds is the possibility of exporting the percentage of speed violators (those driving faster than the applicable speed limit) and offenders (those exceeding the enforcement threshold). The enforcement threshold is 7 km/h above the speed limit for limits up to 80 km/h and 8 km/h for limits of 90 km/h or higher.