The building of roads, bridges and railroads was seen to both a positive and negative impact because...

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European nations used those to extract natural resources and then destroyed them when they left, creating jobs for African people.
African men were hired to build them at higher than normal wages.
Europeans taught the African people how to build this type of infrastructure.
Europeans used them to extract resources, leaving a vast network or routes that were useful to the continent well into the 20th century.

Some of the positive impacts of the building of roads, bridges, and railroads in Africa included creating jobs for African people and providing them with higher wages than they were typically accustomed to. Additionally, these infrastructure projects allowed for easier transportation of goods and people, which could promote economic development and connectivity within the continent.

However, there were also negative impacts associated with these infrastructure projects. European nations often used the roads, bridges, and railroads to extract natural resources from Africa, without necessarily benefiting the local population. Furthermore, when European powers left Africa, they often neglected to properly maintain these infrastructure projects, leading to their deterioration over time. Despite these issues, the building of roads, bridges, and railroads did leave a lasting legacy of transportation networks that were utilized by African nations well into the 20th century.