1. The rule, which has no model number, is described on the case as for "mouvements routiers" (traffic movement).
2. The rule consists, essentially, of three logarithmic scales. The first, in units of distance, is on the upper stock, the second, in units of time is on the slide and and the third is on the lower stock and in units of number of vehicles. The scale on the slide has two slightly unusual features. Firstly it extends for the full width of the slide and can therefore be used by either of the scales on  the stock. The second is that it is in units of time  - with the scale marked in hours and minutes.
3. The upper part of the slide and the stock can be used for speed/time calculations. By setting the 1 hr mark on the time scale against the speed in kilometres/hour on the distance scale, the time for any other distance can be read.
4. The lower part of the slide and stock can be used for movement of convoys. By setting the 1 hr mark on the time scale against the number of vehicles per hour on the lower scale the time for any other number of vehicles can be read directly.
5. The scale for number of vehicles goes up to 3000. That suggest a pretty formidable convey to me - but then if the estimate of the date (see below) is correct this was at the height of the cold war.
6. The instructions (in French) also allow extra for time to develop spacing between vehicles.
7. There is the number 76 marked on the back of the rule - from the look of the rule this could be the date.