Or look to the results of a similar project already completed on the continent. Traffic count might tell you how many lanes you need, but if you already know you are going to build a two lane highway, or a four lane highway now, that won't add much useful information. You are talking about accommodating a new kind of traffic that does not yet exist in your region, so why count tuktuks? You must look to the regional development plans to get an idea about number and type of trucks that will be needed to complete the constructions and to service the long term requirements of the economic development thereby generated and subsequently attracted to the region.
Pavement design is based on the heaviest truck load that will be permitted to use the highway. It does not depend on how many motorbikes, tuktuks, or cars that might use it. The number of trucks within the weight limit that use it, and especially any trucks above the weight limit that use it will increase maintenance requirements.
Drainage design is IMO equally or even more important to highway design as that of the pavement and a lot more of the engineering work than the pavement design itself, which is relatively simple. Spend adequate time on route design to avoid drainage problems and your new highway will be an asset, rather than a maintenance money hog.