Deviated fixed route (DFR) is a type of transit service that combines the features of fixed route and demand responsive services. DFR operates along a fixed route with scheduled stops, but also allows passengers to request deviations from the route within a certain area and time window. DFR can offer more flexibility and accessibility than conventional fixed route service, especially for passengers with mobility impairments or those living in low-density areas. However, DFR also has some drawbacks, such as longer travel times, higher operating costs, and lower reliability. Therefore, it is important to consider the trade-offs and benefits of introducing DFR in transit systems.

One possible criterion for introducing DFR is the level of demand for transit service in a given area. If the demand is too low to justify a regular fixed route service, but too high to rely on a pure demand responsive service, DFR might be a suitable option. DFR can provide a minimum level of service coverage and frequency, while also accommodating the specific needs of passengers who request deviations. DFR can also help reduce the number of transfers and increase the connectivity of the transit network.

Another possible criterion for introducing DFR is the spatial distribution of transit demand in a given area. If the demand is dispersed or clustered in certain locations that are not well served by fixed routes, DFR might be a better option than extending or modifying existing routes. DFR can allow passengers to access destinations that are off the main corridors, such as schools, hospitals, shopping centers, or workplaces. DFR can also help bridge the gaps between different modes of transportation, such as rail, bus, or paratransit.

A third possible criterion for introducing DFR is the temporal variation of transit demand in a given area. If the demand fluctuates significantly throughout the day, week, or season, DFR might be more efficient and responsive than fixed route service. DFR can adjust the frequency and capacity of service according to the actual demand, rather than following a predetermined schedule. DFR can also provide more service during peak hours or special events, and less service during off-peak hours or low-demand periods.

In conclusion, DFR is a hybrid type of transit service that can offer more flexibility and accessibility than fixed route service, but also has some limitations and challenges. The decision to introduce DFR in a transit system should be based on a careful analysis of the demand characteristics, the operational feasibility, and the cost-effectiveness of the service. DFR might be better suited for areas with low to moderate demand, dispersed or clustered demand patterns, and high temporal variation of demand.