Arc routing problems (ARP) are a category of general routing problems (GRP), which also includes node routing problems (NRP). The objective in ARPs and NRPs is to traverse the edges and nodes of a graph, respectively.
The objective of arc routing problems involves minimizing the total distance and time, which often involves minimizing
deadheading time, the time it takes to reach a destination. Arc routing problems can be applied to
garbage collection
Waste collection is a part of the process of waste management. It is the transfer of solid waste from the point of use and disposal to the point of treatment or landfill. Waste collection also includes the curbside collection of recyclabl ...
,
school bus
A school bus is any type of bus owned, leased, contracted to, or operated by a school or school district. It is regularly used to transport students to and from school or school-related activities, but not including a charter bus or transit bus ...
route planning, package and newspaper delivery,
deicing
Deicing is the process of removing snow, ice or frost from a surface. Anti-icing is the application of chemicals that not only deice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or prev ...
and
snow removal
Snow removal or snow clearing is the job of removing snow after a snowfall to make travel easier and safer. This is done by both individual households and by governments and institutions.
De-icing and anti-icing
De-icing is defined as removal ...
with
winter service vehicles that sprinkle
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
on the road,
mail delivery
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
, network maintenance,
street sweeping, police and security guard patrolling,
and
snow plough
A snowplow (also snow plow, snowplough or snow plough) is a device intended for mounting on a vehicle, used for removing snow and ice from outdoor surfaces, typically those serving transportation purposes. Although this term is often used to re ...
ing.
Arc routings problems are
NP hard
In computational complexity theory, NP-hardness ( non-deterministic polynomial-time hardness) is the defining property of a class of problems that are informally "at least as hard as the hardest problems in NP". A simple example of an NP-hard pr ...
, as opposed to
route inspection problems that can be solved in
polynomial-time
In computer science, the time complexity is the computational complexity that describes the amount of computer time it takes to run an algorithm. Time complexity is commonly estimated by counting the number of elementary operations performed by t ...
.
For a real-world example of arc routing problem solving, Cristina R. Delgado Serna & Joaquín Pacheco Bonrostro applied approximation algorithms to find the best school bus routes in the Spanish province of
Burgos
Burgos () is a city in Spain located in the autonomous community of Castile and León. It is the capital and most populated municipality of the province of Burgos.
Burgos is situated in the north of the Iberian Peninsula, on the confluence of t ...
secondary school system. The researchers minimized the number of routes that took longer than 60 minutes to traverse first. They also minimized the duration of the longest route with a fixed maximum number of vehicles.
There are generalizations of arc routing problems that introduce multiple mailmen, for example the k Chinese Postman Problem (KCPP).
Background
The efficient scheduling and routing of vehicles can save industry and government millions of dollars every year.
Arc routing problems have applications in school bus planning, garbage and waste and refuse collection in cities, mail and package delivery by mailmen and postal services, winter gritting and laying down salt to keep roads safe in the winter, snow plowing and removal, meter reading including remote radio frequency identification meter reading technology, street maintenance and sweeping, police patrol car route planning, and more.
Basis
The basic routing problem is: given a set of nodes and/or arcs to be serviced by a fleet of vehicles, find routes for each vehicle starting and ending at a depot. A vehicle route is a sequence of points or nodes, which the vehicle must traverse in order, starting and ending at a depot.
Chinese postman problem
The Chinese Postman Problem (CPP) is aimed at finding the minimum length cycle for a single postman. The CPP requires all edges be traversed once, the rural postman problem (RPP) requires a subset of the edges to be traversed with the minimum length cycle.
Vehicle routing problems/VRP
Arc routing problems impact strategic, tactical, and operational planning decisions. The strategic role of where a depot is placed depends on the most efficient arc route available. The decision of the vehicle fleet size and vehicle types with varying specifications relate to the tactical aspect of arc routing problems in operations research. Routing and scheduling decisions are operational planning decisions in arc routing problems. The operational planning decisions also includes the time that the vehicles are used by workers with staff decisions.
Vehicle routing decisions for the location of a depot depend on the cost of transporting materials over a geographical region. Bodin et. al applied vehicle routing to the dial a ride problem.
Rural postman problem
In some situations, the set of edges that are required is different from the edges in the graph. This is modeled by the Rural Postman Problem (RPP),
where the required edges are a subset of the system of edges.
Algorithms
Finding an efficient solution with large amounts data to the Chinese Postman Problem (CPP), the Windy Postman Problem (WPP), the Rural Postman Problem (RPP), the ''k''-Chinese postman problem (KCPP), the
mixed Chinese postman problem
The mixed Chinese postman problem (MCPP or MCP) is the search for the shortest traversal of a graph with a set of vertices V, a set of undirected edges E with positive rational weights, and a set of directed arcs A with positive rational weights t ...
(MCPP), the Directed Chinese Postman Problem (DCPP),
the Downhill Plowing Problem (DPP), the Plowing with Precedence Problem (PPP), the Windy Rural Postman Problem (WRPP) and the Windy General Routing Problem (WGRP) requires using thoughtful mathematical concepts, including
heuristic optimization methods,
branch-and-bound methods,
integer linear programming
An integer programming problem is a mathematical optimization or feasibility program in which some or all of the variables are restricted to be integers. In many settings the term refers to integer linear programming (ILP), in which the objective ...
, and applications of
traveling salesman problem
The travelling salesman problem (also called the travelling salesperson problem or TSP) asks the following question: "Given a list of cities and the distances between each pair of cities, what is the shortest possible route that visits each cit ...
algorithms such as the
Held–Karp algorithm
The Held–Karp algorithm, also called Bellman–Held–Karp algorithm, is a dynamic programming algorithm proposed in 1962 independently by Richard E. Bellman, Bellman and by Held and Richard M. Karp, Karp to solve the traveling salesman problem ( ...
makes an improvement from
to
.
In addition to these algorithms, these classes of problems can also be solved with the
cutting plane algorithm,
convex optimization
Convex optimization is a subfield of mathematical optimization that studies the problem of minimizing convex functions over convex sets (or, equivalently, maximizing concave functions over convex sets). Many classes of convex optimization probl ...
,
convex hulls
Convex or convexity may refer to:
Science and technology
* Convex lens, in optics
Mathematics
* Convex set, containing the whole line segment that joins points
** Convex polygon, a polygon which encloses a convex set of points
** Convex polytope ...
,
Lagrange multipliers
In mathematical optimization, the method of Lagrange multipliers is a strategy for finding the local maxima and minima of a function subject to equality constraints (i.e., subject to the condition that one or more equations have to be satisfied e ...
and other
dynamic programming methods. In cases where it is not feasible to run the Held–Karp algorithm because of its high computational complexity, algorithms like this can be used to approximate the solution in a reasonable amount of time.
Eulerian circuits
The earliest documented reference to the area of arc routing problems is the classic
bridges of Königsberg challenge, which
Euler
Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries in ma ...
proved to be impossible.
The resident of
Konigsberg, now part of
Kaliningrad
Kaliningrad ( ; rus, Калининград, p=kəlʲɪnʲɪnˈɡrat, links=y), until 1946 known as Königsberg (; rus, Кёнигсберг, Kyonigsberg, ˈkʲɵnʲɪɡzbɛrk; rus, Короле́вец, Korolevets), is the largest city and ...
, wanted to find a way to cross all seven bridges over the river
Pregel without backtracking or retracing their steps, that is crossing each bridge once and only once. In 1736, Euler reduced the problem to a question of nodes and edges and showed that the problem was impossible. In 1873, Hierholzer did more work on the question of closed circuits.
The work on the Eulerian circuits was popularized with Scientific American on July 1, 1953. This work was extended by Meigu Guan, also known as Kwan Mei-Ko at Shangtun Normal College. Meigu Guan was interested in a different question instead of determining a closed circuit. Guan worked to find out a minimum length walk that traversed every edge of the graph at least once. Guan described his goal in 1962: "A mailman has to cover his assigned segment before returning to the post office. The problem is to find the shortest walking distance for the mailman."
Problem types
Arc routing problems (ARPs) differ in their goal and heuristics. However, all of them are known to be
NP-hard
In computational complexity theory, NP-hardness ( non-deterministic polynomial-time hardness) is the defining property of a class of problems that are informally "at least as hard as the hardest problems in NP". A simple example of an NP-hard pr ...
.
Undirected rural postman problem
This problem is named after the postman and his challenge to deliver mail in any order he may choose, but minimizing his costs such as time or travel distance. It is also sometimes called the ''undirected chinese postman problem''. The undirected rural postman problem (URPP) aims to minimize the total cost of a route that maps the entire network, or in more specific cases, a route that maps every edge that requires a service. If the whole network must be mapped, the route that maps the entire network is called a ''covering tour''. In the case where only certain edges need to be mapped, the problem aims to solve the route that optimizes the demands, crossing over into non-required routes a minimal number of times.
Undirected capacitated arc routing problem
The undirected capacitated arc routing problem consists of demands placed on the edges, and each edge must meet the demand. An example is garbage collection, where each route might require both a garbage collection and a recyclable collection. Problems in real life applications might arise if there are timing issues, such as the case in which certain routes cannot be serviced due to timing or scheduling conflicts, or constraints, such as a limited period of time. The heuristics described in this article ignore any such problems that arise due to application constraints.
History
The URPP was first introduced in 1974 and was proven to be an NP-hard problem by
Lenstra and
Kan. The UCARP can be derived from the URPP, and thus is NP-hard as well. In 1981, another pair of computer scientists, Golden and Wong, managed to prove that even deriving a .5 approximation to the URPP was NP-hard. In 2000, Dror published a book describing different arc routing problems.
Windy postman problem and variants
The windy postman problem proposed by Minieka is a variant of the route inspection problem in which the input is an undirected graph, but where each edge may have a different cost for traversing it in one direction than for traversing it in the other direction. In contrast to the solutions for directed and undirected graphs, it is
NP-complete
In computational complexity theory, a problem is NP-complete when:
# it is a problem for which the correctness of each solution can be verified quickly (namely, in polynomial time) and a brute-force search algorithm can find a solution by tryi ...
.
The cost of traveling in one direction is greater when the wind is blowing in your face than when the wind is at your back, and this is the origin of the name Windy Postman problem. The work that it takes to traverse the street in one direction is different than the work it takes to traverse the street in another direction on a windy day.
The windy postman problem is an arc routing problem (ARP) that contains the Mixed Chinese Postman Problem MCPP as a special case.
The problem can be defined in the following manner: "Given an undirected and connected graph G=(V,E) with two non-negative costs
and
associated with each edge
corresponding to the cost of traversing it from i to j and from j to i, respectively, the WPP is to find a minimum cost tour on G traversing each edge at least once."
[ This problem was introduced by Minieka. The WPP is NP-complete in general and can be solved in polynomial time if G is Eulerian, if the cost of two opposite orientations of every cycle in G in same or if G is a series-parallel graph. The Windy Rural Postman Problem (WRPP) is a generalization of the WPP in which not all the edges in the graph have to be traversed but only those in a given subset of required edges. For example, some rural roads are not required for the postman to cross and some roads on steep hills take longer to go up than down.]
The Windy Rural Postman Problem (WRPP) is a generalization of the WPP in which not all the edges in the graph have to be traversed but only those in a given subset of required edges. For example, some rural roads are not required for the postman to cross and some roads on steep hills take longer to go up than down. Consider an undirected graph with two costs and associated with the cost to traverse the edge starting from i and j, respectively. G is the windy graph and we are interested in the subset of edges, or in mathematical symbols, .
If the WRPP includes the additional constraint that a certain set of vertices must be visited—, the problem turns into the Windy General Routing Problem (WGRP). Benavent proposed an integer linear programming formulation and different heuristics and lower bounds for the WRPP.
Benavent et al published an evaluation of several heuristic methods used for solving the WRPP in a few seconds with a deviation no greater than 1% from the lower bound on medium sized graphs. They improved on this with a Scatter Search algorithm that reduced the difference to 0.5%. Scatter Search found solutions that deviated by less than 2% when implemented on networks with hundreds of nodes and thousands of edges.
In real world applications, there are multiple vehicles that can move, which leads to the generalization named the Min-Max K-vehicles Windy Rural Postman Problem (MM K-WRPP). The min–max K-vehicles Windy Rural Postman Problem (MM K-WRPP) is defined as follows: Given a windy graph , a distinguished vertex, , representing the depot, a subset of required edges , and a fixed number K of vehicles, the MM K-WRPP consists of finding a set of K tours for the vehicles in such a way that each tour starts and ends at the depot and each required edge is serviced by exactly one vehicle. The objective is to minimize the length of the longest tour in order to find a set of balanced routes for the vehicles. Some real-life applications of routing problems with min–max objectives are school bus routing (Delgado and Pacheco 2001), the delivery of newspapers to customers (Applegate et al. 2002) and waste collection (Lacomme et al. 2004).
The best MM K_WRPP algorithm was very close to the minimum solution with 2 and 3 vehicles, less than 0.4% on average. The gap increases to about 1.00% and 1.60% at 4 and 5 vehicles.
According to Dussault et al and Benavent et al, a metaheuristics multi-objective simulating annealing algorithm (MOSA) can solve the different contraints imposed on the WRPP. The WRPP is an important Arc Routing Problem which generalizes many of the single-vehicles Arc Routing problems. In real applications of math, a solution that minimizes the total costs of all vehicles route and the length of the longest tour is preferable. It's hard to be in a location where your package is always hours late. We should start with the assumption that several vehicles with a specific measurable capacity to serve customers is more realistic than one vehicle with unmeasurable infinite capacity. Rabbani et. al measured the performance of MOSA algorithms and models using a multi-objective development of Cuckoo search—developed by Yang et al, also referred to as Multi-objective Cuckoo Search and abbreviated by MOCS. They concluded that MOSA methods were more efficient than MOCS methods. In the future comparisons with other meta-heuristic methods could be researched, including Non-dominated Sorting Genetic Algorithm (NSGA- ), multi-objective particle swarm optimization algorithm (MOPSO) and multi-objective Imperialist Competitive Algorithm.
In the Windy Postman Problem (WPP) model, the cost of going in one direction is different than the cost it takes to go in the other direction. For example, if the wind is blowing down the street it takes more time and energy to go against the wind than with the wind. Another example of the WPP is the cost of plowing uphill is greater than the cost of plowing downhill. This is modeled by a variant studied by Dussault et al, the Downhill Plowing Problem (DPP).
A branch and cut algorithm was published by Angel Corberan for the windy postman problem. The algorithm is based on heuristic and exact methods for manipulating odd-cut inequality violations.[
]
Applications
Various combinatorial problems have been reduced to the Chinese Postman Problem, including finding a maximum cut in a planar graph and a minimum-mean length circuit in an undirected graph.
Snow plows
In winter a common question is what set of routes has the smallest (minimum) maximum route length? Typically, this is assessed as an arc routing problem with a graph. The time it takes to travel a street, known as deadhead time, is faster than the time it takes to plow the snow from the streets (or deliver mail or drop off packages). Another aspect that must be considered when applying arc routing to snow plowing is the fact that on steep streets it is either difficult or impossible to plow uphill. The objective is a route that avoids plowing uphill on steep streets that completes the job faster by maximizing the deadhead time to get the location. This was modeled with a heuristic algorithm that approximates a lower bound by Dussault, Golden and Wasil. This is the Downhill Plow Problem (DPP). Snow teams prefer to plow downhill and deadhill uphill. This problem assumes that the conditions are severe enough that the streets are closed and there is no traffic.
The Downhill Plowing Problem ignores the Plowing with Precedence Problem (PPP), which is built on the reasonable assumption that if the snow is too deep the snow plow cannot deadhead an unplowed street. The DPP makes the assumption that the snow level is low enough that the streets that are not plowed can be deadheaded, but that the snow is deep enough that there is no traffic. If there is traffic on the roads, the assumption that it is impossible to plow uphill can no longer be held. The simulation for the DPP deadheaded unplowed street about 5% of the time, which is a topic for future graph theory and arc routing research.
Considering an undirected graph where is the set of vertices and nodes and is the set of arcs. Each arc represented by has four costs: , defined as the cost of plowing from to , , the cost of plowing from to , , the cost of deadheading from to , and , the cost of deadheading from to . The setup assumes that has a higher elevation , which leads to the statement: . In practice, downhill plowing time is two times as efficient as uphill plowing and deadheading is twice as efficient as plowing. The algorithm finds routes will each begin and end at the depot , plow the arc two times because the left side and right side of the street take two passes to plow.
The best solution will minimize the maximum route length. Dussault, Golden, and Wasil found an algorithm that did not exceed the lower bound by 5.5% in over 80 test runs. The deviation increased as the complexity of the model increased because there are more unoptimized approximations than optimized approximation as the model grows. An improvement on Dussault et. al's DPP algorithm might have penalties for making U-turns and left hand turns, or going straight across an intersection, which take additional time and pushes snow into the middle of the intersection, respectively. (see The Directed Rural Postman Problem with Turn Penalties problem, often referred to as the DRPP-TP below).
''k''-Chinese postman problem (''k''-CPP)
The ''k''-Chinese Postman can be stated as follows: "given a connected edge-weighted graph ''G'' and integers ''p'' and ''k'', decide whether there are at least ''k'' closed walks such that every edge of ''G'' is contained in at least one of them and the total weight of the edges in the walks is at most ''p''?" The process of obtaining the solution to the ''k''-CPP is NP complete. Gutin, Muciaccia, and Yeo proved in 2013 that the ''k''-CPP is fixed-parameter tractable. The authors prove the ''k''-CPP admits a kernel with vertices and the directed version of the ''k''-CPP is NP complete.
Rural postman problem (RPP) and generalizations
The rural postman problem (RPP) makes some routes mandatory and absolute but the person traversing the graph does not have to go in one particular direction. The RPP is NP hard and complete, in the same way that the kCPP, the DPP, the PPP, are NP hard. Benevant studied a generalization of this named Directed Rural Postman Problem with Turn Penalties (DRPP-TP). Benevant's algorithm approximated the solution by transforming the DRPP-TP into an asymmetrical traveling salesman problem (ATSP).
Heuristics and algorithms
Most algorithms require a pre-processing of the graph, which simplifies the initial graph by removing all edges that are not in the shortest path between two required edges. Another simplification that the pre-processing adds is that it transforms the shortest path between 2 required edges into a single, non-required edge, regardless of the number of edges in the path, provided that there were no required edges in the path.
Once the pre-processing is done, the problem can be generalized into a convex hull
In geometry, the convex hull or convex envelope or convex closure of a shape is the smallest convex set that contains it. The convex hull may be defined either as the intersection of all convex sets containing a given subset of a Euclidean space ...
problem, with the edges being the points of the hull. The convex hull problem can be solved through linear programming or through convex hull algorithms, but the process of finding the convex hull is an exponential problem.
Methods of solving the URPP after the pre-processing is done consist of the ''cutting plane algorithm'' and the ''branch & cut methodology''.
Complexity
This is a list of computational complexities for different arc routing problems.
List of arc routing variants
External links
Arc Routing Problems Search page at Lancaster University
Trends in Arc Routing
See also
* Chinese postman problem
In graph theory, a branch of mathematics and computer science, Guan's route problem, the Chinese postman problem, postman tour or route inspection problem is to find a shortest closed path or circuit that visits every edge of an (connected) undire ...
* Vehicle routing problem
The vehicle routing problem (VRP) is a combinatorial optimization and integer programming problem which asks "What is the optimal set of routes for a fleet of vehicles to traverse in order to deliver to a given set of customers?" It generalises ...
* Travelling salesman problem
The travelling salesman problem (also called the travelling salesperson problem or TSP) asks the following question: "Given a list of cities and the distances between each pair of cities, what is the shortest possible route that visits each cit ...
* Eulerian path
In graph theory, an Eulerian trail (or Eulerian path) is a trail in a finite graph that visits every edge exactly once (allowing for revisiting vertices). Similarly, an Eulerian circuit or Eulerian cycle is an Eulerian trail that starts and ends ...
* Capacitated arc routing problem In mathematics, the capacitated arc routing problem (CARP) is that of finding the shortest tour with a minimum graph/travel distance of a mixed graph with undirected edges and directed arcs given capacity constraints for objects that move along the ...
* List of graph theory problems
In mathematics, graph theory is the study of ''graph (discrete mathematics), graphs'', which are mathematical structures used to model pairwise relations between objects. A graph in this context is made up of ''Vertex (graph theory), vertices'' ( ...
* Snow plow routing problem The snow plow routing problem is an application of the structure of Arc Routing Problems (ARPs) and Vehicle Routing Problems (VRPs) to snow removal that considers roads as edges of a graph.
The problem is a simple routing problem when the arriv ...
* Chinese Postman Problem Complexity List
References
{{reflist
Routing algorithms
Travelling salesman problem