Bridges, like many man-made structures, can have a significant negative impact on the environment. Since many bridges are built to cross rivers, and since rivers are often a diverse ecosystem, the impact of a bridge on the surrounding environment must be taken into account. When building bridges and other highway structures, environmental assessments are often required, so that there can be assurances that the bridge will not adversely affect the local environment. Cables, for instance, a popular feature of many bridges, kill birds who make their home on the river.
Highway construction projects were common in past decades, but in the 21st century there are not all that many new highways being built, so most of the bridges that are built today are replacing older, existing bridges. Old bridges are often improved, rather than building new ones from scratch, and this can cause traffic delays for those who use the bridge, as lanes will often be closed to allow room for construction. Bridge construction often takes place at night and early in the morning, to help reduce the impact of bridge construction on traffic.
Another common effect of bridge construction is the introduction of tolls and taxes to pay for the new bridge. Bridges are expensive, after all, and toll booths in particular are often seen as a good way to help pay for them because it is the people who use the bridge that end up paying for it. Toll booths may be removed after a monetary goal is reached, or they may be left in place to help pay for future improvements and construction.
The point of building a new bridge is to, usually, improve upon an old bridge. The old bridge may have become unsafe, and in need of repairs, but it may also have become too small for the amount of traffic using it, causing delays. Bridges are often augmented with new lanes, or new, larger bridges are constructed to allow for more traffic. One of the long term effects of a bridge construction project is an ultimate improvement in bridge performance and safety.
