A negative externality is a cost that is caused by some economic activity but which is not paid by the entities that are directly involved in the activity. In the case of road building, this would be costs that are incurred by people who are not building or using the road.
A major externality associated with road construction is the pollution that is caused by the use of automobiles. As more roads are built, more cars use them. As more cars use them, more pollution occurs and impacts even people who do not use the roads. A similar impact would be excessive noise. If, for example, a large road is built near your house, you will bear the cost of having to deal with all the extra noise even if you do not use the road yourself.
Air pollution and noise pollution, then, are two major negative externalities that are associated with the building of roads.
It would not be wrong to say that one of the negative externality of constructing roads is an increase in air pollution, though it is not right to say that it is the construction of new roads that leads to an increase in the number of new vehicles. New roads are constructed to facilitate the movement of a larger number of vehicles that are already present. This may actually help to reduce air pollution as the vehicles can travel on the new roads at speeds which are closer to that at which the efficiency of their engines is the highest.
The cause of the increase in air pollution is the fact that a large number of trees may have to be cut when new roads are being laid. The new roads may also have to be built through wooded areas which results in the wildlife getting disturbed. The construction of roads requires raw materials like tar, cement, concrete, etc, the production of which is a highly polluting negative externality.