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Starting from AccessMod version 5.2.1, speeds of zero are allowed in the scenario table. A speed of zero translates into a full barrier status for all concerned cells. However, when a facility falls into a "Zero speed" cell, it will not be taken into consideration in the analysis, in a similar way than facilities falling on barriers (NoData) cells. In this case, such facilities will have a “Yes” label in the “amOnZero” column of the "Selected facilities" table. These facilities will either have to be unselected or their coordinates modified before being able to run the analysis.
Speed corrections due to topography for walking and bicycling modes are computed through the following steps in AccessMod:
- Slopes between a given cell and its eight neighbors are computed first.
- A correction based on slope and on the transportation mode (walking or bicycling) is computed using the functions reported in the following sub-sections.
- This correction is applied to the speed attributed to travelling between any two neighboring cells, as inputted by the user in the travelling scenario table.
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Each land cover category (class) found in the merged land cover distribution grid (see Section 3.3.1.4) must have a corresponding entry in the travelling scenario table. If this is not the case, an on-screen warning message asking the user to revise the table will appear in the validation section of the analysis window. The user will not be able to start the analysis until a speed is attributed to each class. Such change can be performed directly in AccessMod. WALKING and BICYCLING corrections are only applied when a DEM, and therefore an anisotropic (slope-dependent) approach, is used in the analysis (see section 3.3.1.2). |
Speed correction for the walking model
This correction uses Tobler's formula (Tobler, 1993) that links walking speed with the slope of the terrain through the following formula:
where V is the corrected walking speed in kilometers per hour (Km/h), VF is the walking speed on a flat surface (given by the user-defined travelling scenario delineated here above), and S is the slope in hundredth of percent.
A graphical representation of this relation is shown in
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