What's the proper method of calculating the depreciation of a Transportation Company?

Regarding Depreciation IAS16 is very flexible and give option how you want to depreciate you assets but make sure that it further says that It should reflect the best estimate.
And with the passage of time if you realize then you can change this estimate and treat it under the requirements of IAS8 as a change in accounting estimate and apply prospectively.
Now I'll discuss the depreciation method for transport company: Before applying depreciation please make sure one thing "How you use your vehicles?" If you are using very excessively then you can apply depreciation by 'number of kilometer traveled' If these are used normally then you can apply what is industry norm which may be any of depreciation methods.

There are two methods: Straight line and Double declining balance

 

Use Declining Balance Method, it matures your assets faster

Use Straight line, if creates a constant deduction and increases asset life

Straight Line Depreciation Method The simplest and most commonly used depreciation method, straight line depreciation is calculated by taking the purchase or acquisition price of an asset subtracted by the salvage value divided by the total productive years the asset can be reasonably expected to benefit the company (called "useful life" in accounting jargon).

depriciation is calculated on assets n transportation company has to take depriciation on its assets depriciation can be calculated by straight line method or deminishing balance method

the question is not clear....
Transportation is expenses without depreciation.
If you have bus or cars for transportation so you will make depriciation for the asset.