A barrier is a feature that can impede mobility and comes in three forms:
- Absolute barriers are geographical features that prevent mobility, making it impossible to go beyond the barrier in its current form. They must either be bypassed or overcome by specific infrastructures. For instance, a river is considered as an absolute barrier for land transportation and can only be overcome if a tunnel or a bridge is constructed. A body of water forms a similar absolute barrier and could be overcome if ports are built and a maritime service (ferry, cargo ships, etc.) is established. The barrier will be overcome by changing mode. Conversely, land acts as an absolute barrier for maritime transportation, with discontinuities (barriers) that can be overcome with costly infrastructures such as navigation channels and canals.
- Relative barriers are geographical features that impose a level of friction on a mobility. Mobility is possible but comes at a cost that varies according to the level of friction. In turn, this friction is likely to influence the path (route) selected to link two locations. Topography is a classic example of a relative barrier that influences land transportation routes along paths having the least possible friction, such as plains, valleys and low gradient slopes. For maritime transportation, relative barriers generally slow down circulation such as straits, channels or ice. Adverse weather conditions can be a relative barriers for air transportation, imposing detours around storm fronts.
- Arbitrary barriers are non physical effects on a movement that can be linked to a specific area, more than often a jurisdiction. They are called arbitrary since they are the outcome of human decisions and activities. For instance, a change in international jurisdiction involve going through customs procedures, adding costs and delays. This could also involve different cost structures such as taxes and tolls, as well as operating conditions (e.g. speed limits or weight restrictions). Some areas could be subject to more stringent environmental regulations, even forbidding the use of specific modes or the construction of infrastructure. Another arbitrary barrier concerns risk, as some areas are subject to conditions such as political instability, that may increase to risk of theft and plunder, and therefore the cost of servicing these areas.