Slavik M and Bosman J. Traffic loadings estimated from counts. 3rd International Road Federation/South African Road Federation Regional Conference for Africa. Durban, South Africa, September 2006.

 

Abstract

The most accurate way to determine traffic loadings would be to weigh heavy vehicles statically. This is unpractical for obvious reasons and weigh-in-motion (WIM) measurement of traffic loadings are reverted to as a “second best” option. WIM traffic data, however, is very expensive and is only available at a limited number of counting stations. The aim of this paper is to develop a methodology to estimate traffic loadings from normal loop counts as a “third best” option. In this option the number of long trucks (LT) (18m or longer) expressed as a percentage of all the trucks on a road was used to stratify roads into three groups ( roads with LTs less than 35 %, LTs between 35 % - 55 %, and LTs over 55 %). The law enforcement strength (strong and weak) was used to as a further stratification of the three road groups. Five different traffic loading cases were developed and WIM measured axle load distributions were constructed for each of these. Although the correlation between the axle loads and percentage long trucks is not very good yet, this method can be used by designers in the absence of better traffic loading data. Standard specifications for traffic data collection by the South African National Roads Agency Limited (SANDAL) may improve the accuracy of estimated traffic loadings from ordinary traffic counts in future.

 

TABLE 1. Five Traffic-loading Model WIM Stations

 

 

TABLE 2. Traffic and Sample Sizes at the Five Traffic-loading Model Stations in 2005

 

 

TABLE 3. Key Figures of the Five Traffic-loading Types in 2005

 

 

Acknowledgements

 

The authors wish to express their gratitude to:

 

NTRV (Northern Toll Road Venture, the N1 Toll Road Concessionaire),

N3TC (N3 Toll Concession, the N3 Toll Road Concessionaire),

TRAC (Trans African Concessions, the N4 Toll Road Concessionaire),

Bakwena (the N4 Platinum Toll Road Concessionaire), and

SANRAL (South African National Roads Agency Limited).

 

for their kind permission to use their traffic loading data.

 

Further acknowledged is the financial assistance of the The Concrete Institute that made extensive WIM data re-processing possible.