Traffic Data 1

[kennydalglish]

I am designing a highway to the Guinea Border in West Africa which when completed will generate traffic including container trucks and other articulated and heavy vehicles. The existing condition of the road is such that 99.8% of the traffic is from motorbikes and only four wheel drive vehicles can use it. We intend to carry out classified traffic counts and OD surveys etc as per the TOR.

I want to know how to carry out a pavement design given the limited traffic data.

 

[IRstuff]

Does that matter? If the intent is to accommodate container trucks and other large vehicles, then it needs to be designed for that. It's current condition obviously can't support either the intended traffic nor any projection of what the traffic patterns might look like afterwards.

When interstate 105 was completed in Los Angeles around 1994, it was intended to draw traffic away from a city street and for the first month after opening it was a lovely drive across LA. Afterwards, it not only drew the street traffic, but also from other congested east-west freeways and it became painfully obvious that it was woefully underdesigned. Of course, being LA, there probably is no way it can be overdesigned, because freeways and highways are like the vacuums that nature abhors. No matter how robust it's designed or how many lanes are designed, demand is whatever the traffic will bear.

I would suggest that you look at the overall traffic on adjacent and complementary roads and try to assess how much of that traffic will decide to move to a better road, and how much additional traffic will be generated that doesn't even exist, because the existing infrastructure is too poor. Additionally, I would suggest that you anticipate that the road will need to be widened in the near future.

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[1503-44]

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.