The commonplace technology for bridges is either concrete or steel. These techniques require a high input of industrial materials and a low amount of labour. However, in rural Tanzania, it is very expensive to use industrial materials, while labour is cheap.
The stone arch technology capitalises on this fact by using readily available local materials in a labour intensive process. This leads on average to a reduction of 80 to 85% in construction costs, and a 50 to 80% reduction in carbon emission due to lower use of cement and reinforcement steel, when compared with conventional construction techniques.
Masonry bridges also avoid some of the disadvantages of conventional alternatives, such as the use of steel or concrete culverts. These frequently break during rough transport over bad roads from industrial centres and, because they are relatively light, they run the risk of being washed away during floods. In addition, such prefabricated elements are sometimes stolen. People dig them out and try to resell them to make a bit of extra money.
Stone arch bridges have been built for more than 2000 years and some of the early structures are still used today. The Fabricius bridge in Rome for example has two arches spanning 24m each and dates from 62 BCE
. Both the design and execution rely on simple rules of thumb that have been tried and tested through time. This avoids the need for complicated design and precision sensitive execution of steel and concrete bridges.