In continuation of a set of types of problems in fleet
management area, Urban public transport and dial-a-ride are more recent.
Urban public transport deserves to be called a class of
problems by itself. It consists of determining ways to provide good quality of
service to passengers with finite resources and operating costs. Their planning
process often involves 1. Network route design, 2. Frequency setting and
timetable development, 3. Vehicle scheduling and 4. Crew scheduling and
rostering. Some state-of-the-art models involve tuning the routing and
scheduling with minimization of passenger cost functions. Metaheuristics
schemes that combine simulated annealing, tabu, and greedy search methods serve
this purpose. One of the distinguishing features of this problem space is that
customers often formulate two requests per day, specifying an outbound request
from pick-up to drop-off and an inbound request for the round trip. Another
feature is that the quality of service needs to be maximized while minimizing
operating costs incurred to satisfy all the requests.
Dial-a-ride
transport is a move toward more economical and greater flexibility of transport
services. Demand responsive transportation systems require the planning of
routes and customer pick-up and drop-off scheduling on the basis of received
requests. It must deal with multiple vehicles with limited capacity and time-windows.
The problem of working out optimal routes and times is referred to as the
Dial-a-ride problem. As with many problem spaces in fleet management, this can
be treated as NP-hard combinatorial optimization problem. Attempts to develop
an optimal solution has been limited to simple and small-size problems.
Such a
service may operate in a static or dynamic mode. In the static settings, all
the customer requests are known beforehand, nd the system solves a tour each
vehicle must make within the constraints of the pick-up and drop-off time
window and minimizing the solution cost. In the dynamic mode, the customer
requests arrive over time to a control station and the solution may change over
time. Processing must also keep up with the incoming rate without interfering
with the optimization cycle at the end of the service. The goal is two-fold:
reduce overall costs and improve quality of service to customers. Several
algorithms have been tried for this purpose. They include tabu search
heuristics, dynamic programming, branch and cut, a heuristic two phase solution
method, genetic algorithms and variable neighborhood search.
Even the
users can be differentiated as well as the transportation modes. Some
meta-heuristics such as vehicle waiting time with passenger onboard can also be
used with branch-cut algorithms for this purpose.
Studying
quality of service in this context has evolved into models which use various
measurement scales. The quality of service provided by organizations also
depend on the type of organization and the operational rules used.
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