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Topology dependence of ondemand ridesharing
Applied Network Science volume 5, Article number: 49 (2020)
Abstract
Traffic is a challenge in rural and urban areas alike with negative effects ranging from congestion to air pollution. Ridesharing poses an appealing alternative to personal cars, combining the trafficreducing ride bundling of public transport with much of the flexibility and comfort of personal cars. Here we study the effects of the underlying street network topology on the viability of ride bundling analytically and in simulations. Using numerical and analytical approaches we find that system performance can be measured in the number of scheduled stops per vehicle. Its scaling with the request rate is approximately linear and the slope, that depends on the network topology, is a measure of the ease of ridesharing in that topology. This dependence is caused by the different growth of the route volume, which we compute analytically for the simplest networks served by a single vehicle.
Introduction
The increasing demand for mobility in modern urban, suburban and rural areas presents a wide range of ecological and logistic challenges. While urban areas struggle with traffic jams, air pollution and parking space shortages (Guerreiro 2018; NYC Department of Transportation 2018), rural areas are often unable to provide accessible and frequent public transport. The recent rise of the sharing economy (Belk 2014; Cohen and Kietzmann 2014; Kamargianni et al. 2016; Greenblatt and Shaheen 2015) has brought up ridesharing as a possible answer to all of these problems. Ridesharing poses an appealing alternative to personal cars, combining the trafficreducing ride bundling of public transport with much of the flexibility and comfort of personal cars (Spieser et al. 2014; Zhang and Pavone 2016; Barbosa et al. 2018; Macharis and Keseru 2018; Vazifeh et al. 2018). Intelligent ondemand ridesharing services are hoped to reduce the ecological footprint associated with individual mobility by dynamically bundling rides together, reducing the amount of vehicles necessary for the same number of rides (Tachet et al. 2017; Santi et al. 2014; Sorge et al. 2015; Sorge 2017).
However, the complex behaviour of such dynamic dialaride problems (DARP) (Berbeglia et al. 2010) is not yet fully understood. Recent studies have examined the dynamical behaviour of specific ridesharing strategies analytically (Herminghaus 2019; Molkenthin et al. 2019) or in simulations (AlonsoMora et al. 2017; Ma et al. 2013; Agatz et al. 2011; Horn 2002). However, the general scaling behaviour, or dependence on street network topology and request patterns are not currently understood. Such an understanding would be necessary to compare different dispatching strategies and network settings and make informed decisions about which dispatching strategy works best for a particular network.
Here we study the effects of the underlying street network topology on the viability of ride bundling analytically and in simulations in the lowdensity limit by studying the performance of a single vehicle. We find that for finite request rates and vehicles not restricted by capacity, there is always a quasistationary regime of operation, varying in waiting time and typical vehicle occupancy. We develop a probabilistic description of the steadystate route length, relying on routevolume, a topological characteristic of a network that we define, and use this to derive the equilibrium stoplist length. Based on this we show the scaling of the steadystate stoplist length with the dimensionless request density to be linear with a slope depending on the topology. The dependence of the routevolume on the stoplist length can be approximated explicitly for some simple topologies (ring and star) and numerically otherwise.
We apply this analysis to unweighted realworld street networks. The general layout of urban centers is predominantly gridlike in structure, whereas rural areas appear to be best described as interconnected rings with long stretches of unbranching streets. This leads to the surprising effect that, while the request density of cities tends to be better suited for ridesharing, the topologies show the opposite trend with rural areas allowing easier bundling. This is particularly important as cities already have well functioning public transport options, which have proven to be impractical in less densely populated areas.
Model
We use a simplified model to study ridesharing, in which the street network is represented as a simple graph with N nodes. Requests are placed according to a request pattern P_{i,j,t} and serviced by a single vehicle, where a request is an ordered pair of a pickup node i and a dropoff node j drawn from the request pattern at time t. The request pattern used throughout this manuscript is uniform across all node pairs and time to facilitate analytical treatment. Related studies have found, however, that such simplifications only have small impacts on the overall behaviour of the system (Molkenthin et al. 2019). New requests arrive according to a Poisson process with an average time Δt between requests to be included in the route according to a dispatcher algorithm. The capacity of the vehicle is presumed to be unlimited to allow for the analysis of a wide range of request rates. Introducing a finite capacity would lead to rejections and thus effectively an upper bound for the request rate, beyond which the system overloads. The dispatcher algorithm checks if the request’s pickup node can be inserted to the existing stop list without incurring any detour. If this is the case it checks if the dropoff node can also be inserted to the existing stop list without any detour, otherwise it is appended at the end of the stop list. If the pickup cannot be inserted with zero detour, then both the pickup and the dropoff are appended right after each other at the end of the stop list.
We introduce the dimensionless request rate x in order to compare system properties across network topologies
where 〈l〉 is the average length of the requested ride and ν is the bus speed.
A request rate of x=1 means that the vehicle covers a distance of 2〈l〉 between two requests, where 2〈l〉 is the expected distance from the endpoint of the route to the new pickup and from there to the new dropoff. Therefore even without ridesharing, rides are on average completed within one Δt and even a taxi system would be able to serve them all, operating at maximum capacity. For x>1 the taxi can no longer serve the system and waiting times diverge, in this case the ridesharing system transports x times as many passengers as the taxi could.
To quantify passenger satisfaction in a ridesharing system, we investigate the service time t_{s}, i.e. the time it takes from placing the request until being delivered at the requested dropoff location. The number of planned stops in the system, the stop list length n, on the other hand serves as a measure of performance from the perspective of the system as a whole.
Starting from an empty vehicle in a random position, we subsequently generate random requests with pickup and dropoff nodes chosen uniformly randomly from the nodes of the graph, that are then included in the route by the dispatcher algorithm and served at constant velocity. This is repeated until the steady state is established and performed on a range of different network topologies (star, ring, grid, city layouts) and request rates (0<x<40). Results of the simulations on a ring with 10 nodes are shown in Fig. 1. The complete simulation code is available in (Manik 2020).
Analytics
We analytically derive approximations for the stop list length n in the steady state, by solving the evolution equation for the length of the planned route after r insertions L_{r}.
Where l^{+}(n) is the average added length per request, νΔt is the distance driven in between requests and the discrete time parameter r counts the requests.
The planned route length reaches its equilibrium at
where we used the definition of x from Eq. 1.
In case of a taxi system the added length is independent of n at two times the average shortest path length in the network
as new segments are simply added to the end. If x>1, the system no longer has an equilibrium as the route length keeps growing, if on the other hand x<1, the taxi has time between subsequent rides, in which it stands still, lowering the average velocity.
In a ridesharing system with a sensible dispatcher algorithm, however the length of the added segments depends on the planned route. In our model for example, as the current length of the route n grows, the probability that a new request’s pickup and/or dropoff being already included in the route increases, resulting in a smaller l^{+}(n).
We take the added length to be the average over three possibilities:

a
Both, pickup and dropoff node are already on the route.

b
The pickup node is on the route but the dropoff node is not.

c
The pickup node is not on the route.
In case a) no length is added to the route, in case b) the average added length is 〈l〉 and in case c) the route gets longer by an average of 2〈l〉. This means that
where P_{b} and P_{c} are the probabilities of b and c respectively.
To evaluate the probabilities, we introduce route volumeV as the number of nodes that can be reached within the stop list without a detour (illustrated in Fig. 7 in Appendix A). The probability for the requested pickup node to be on the route depends on the expectation value 〈V〉 of the volume of the route. The volume depends on the length of the stoplist as well as the topology of the underlying network. As the requested dropoff point has to be on the route after the pickup, there is a second relevant route volume V_{rest}, and its expectation value 〈V_{rest}〉(n), the average volume of the route after the pickup. Assuming the position of the pickup is uniformly randomly located somewhere along the route, the fact that the insertion of the dropoff is always after the pickup leads to (we employ a simplifying assumption that the pickup is equally likely to be inserted at any position in the stoplist)
We see in Fig. 2ab that V_{rest} so computed is slightly lower than the actual value. This is because in reality, the pickup location is slightly more likely to be at the second half of the stop list than the first half. We demonstrate this in Fig. 9 in Appendix A.
Furthermore, we note that the function 〈V〉(n) is always monotonously growing with n and asymptotically approaching N. We thus express 〈V〉_{rest}(n) for large n as
where \(\alpha =\sum _{k=0}^{n1} \left [1\langle {V}\rangle (k)/N\right ]\), if this limit exists. Note that \({\lim }_{n\to \infty }1\langle {V}\rangle (n)/N=0\), and if in addition we know that 1−〈V〉(n)/N goes to 0 faster than 1/n, then α is guaranteed to exist. In this case, α is a constant, depending only on the volume growth in a particular network, so 〈V〉_{rest} approaches N with n^{−1}. We demonstrate in Fig. 2c that at least for rings and stars, this assumption holds.
Using this, we can express the probabilities for the three insertion types:
This is shown for a number of different networks in Fig. 3.
In a ring of length N the expected route volume 〈V〉_{ring}(n) for a stop list of length n is given by the recursive relation
A detailed derivation (22) is given in the Appendix A. This approximation holds very well as shown in Fig. 2a.
In a star with N nodes, the number of nodes on the route is approximately equal to the number of unique random draws. This is given by
This approximation does not account for the special role of the center point of the star, which is always on the route, as soon as the stoplist contains two or more nodes. Nonetheless it holds reasonably well, as shown in Fig. 2b.
Results
We have now gathered all necessary input for computing the expected steadystate stoplist length. Inserting the approximated volumes from Eq. 9 and Eq. 10 (or using volumes extracted from the simulation if no such approximation is available) into the probability functions from Eq. 8 and the approximation of the second volume from Eq. 6 to then insert into the steady state added length from Eq. 5 and solving for n, we find an approximately linear rise of the stop list length with the dimensionless request rate x (see Fig. 4a and b). While the general trend is captured well, the slopes of the analytical approximations underestimate the actual value. This is due to the overestimation of 〈V〉_{rest}(n) by the assumption of uniformly random insertion of pickups in the stoplist, limitations of which assumption is demonstrated in Fig. 9 in Appendix A. The discrepancy therefore largely disappears when measured values are used for 〈V〉_{rest}(n), as in Fig. 4c
Independently of the exact form of 〈V〉(n) we can exploit the asymptotic behaviour of 〈V〉(n) and set \(\frac {\langle {V}\rangle }{N}\rightarrow 1\), leading to (using (3) and (5))
where we inserted the probabilities from Eq. 8 into Eq. 5 and set \(\frac {\langle {V}\rangle }{N}\rightarrow 1\).
In this we use the expression for α from Eq.7 and solve for x, giving
where \(\alpha ={\lim }_{n \rightarrow \infty }\sum _{k=0}^{n1} \left [1  \langle {V}\rangle (k)/N\right ]\) is computed from the analytical expressions of 〈V〉(k) or directly from simulated 〈V〉 and 〈V〉_{rest}. In Fig. 4c the estimated results for α are inserted in Eq. 12 and plotted with the directly simulated n(x). For each topology and request rate, 10000 requests were simulated, with the origin and destination of each request drawn uniformly randomly from the nodes (Manik 2020).
We find values of α≈11.4 for the ring, α≈6.4 for the line, α≈28.2 for the grid, α≈35.4 for the triangular grid and ≈186.0 for the star. The resulting lines capture the behaviour of the curves reasonably well.
The star graph has by far the steepest curve, indicating the worst layout for ridesharing. This was expected as there is only one point that is on the way while all other nodes are detours. The grids perform slightly better, as there are multiple routes between any two points. The ring and line essentially represent the ridesharing in an elevator, which works without much route adjustment by simply going up and down and collecting whomever is going in the current direction of the elevator.
This shows that ride sharing on a ring or line is very natural and typically possible, while almost no two distinct rides can be bundled on a graph with star topology (see Fig. 4).
We have applied the methods to real street networks to assess their respective ridesharing feasibility and compare rural, urban and suburban areas. To this end we have extracted street networks from the freely available open dataset curated by the OpenStreetMap project (OpenStreetMap contributors 2020) using the opensource library OSMnx (Boeing 2017) and translated the weights into a corresponding number of equally long links, since our method is meant for unweighted graphs. First, if two nodes are less than 100 m apart, they are collapsed into one, then all links longer than 400 m were subdivided by adding additional nodes. This procedure ensures that link lengths are approximately uniform. This procedure is further illustrated in Appendix B and Fig. 11. In the resulting graph the distribution of edge lengths are rather sharply peaked around 400 m. For example, in case of the Berlin network in Fig. 5, edge lengths have mean 310.19 m.
These alterations leave the shortest path lengths in the networks largely unaffected. In each of Berlin, Göttingen and Harz, the average shortest path length remains within 20% of the original value (when comparing the original weighted average shortest path length in meters against the new unweighted shortest path length multiplied by the average link lengths in meters). In urban networks, the original street lengths are largely homogeneous, leading to few added intermediate nodes. In rural areas, on the other hand, street lengths in between settlements are far larger than those within villages, leading to large numbers of added nodes. As a result, street networks in the city, as shown in Fig. 5a resemble a grid, whereas those in the countryside resemble a loose mesh.
In order to distinguish effects of network size N and the interaction structure on ridesharing efficiency, we compare networks of equal size and link length. This is done by selecting geographical regions to include comparable numbers of nodes at the same coarse graining criteria: collapsing node pairs that are <100 m distant, then introducing intermediate nodes along edges to achieve a target edge length of 400 m. By suitably choosing the geographical bounding boxes, we generate networks with ≈500 nodes and simulate 10000 requests in each case. The resulting stoplist length is plotted in Fig. 5 over the dimensionless request rate, assuming a uniformly random distribution of requests, as in the case of the artificial networks. We observe linear behaviour with the slope depending on the underlying network, just like we did for synthetic networks in Fig. 4.
In particular we find that the gridlike structure of the large city (Berlin) leads to a far steeper slope than the loose mesh of the rural area (Harz), or a single town surrounded by smaller villages (Göttingen), indicating that the rural topology may be more suitable for ridesharing, when the request pattern is uniformly randomly distributed.
We furthermore explore the effect of network size, while keeping the network structure constant by comparing ridesharing between different coarsegraining levels of the same region of Berlin: we collapse all node pairs <200 meters distant, then introduce intermediary nodes to achieve target edge lengths of 200,600 and 800 meters. Despite the normalization of the request rate with the average shortest path length, we find the slope to depend on the network size, as well as structure, as shown in Fig. 6. This means if the same geographical area is modelled as a sparser network (i.e. with fewer nodes), it shows higher ridesharing efficiency than a denser network. This happens because in a sparser network, it is more likely that the shortest path between the origin and destination of two different requests will have an overlap. Indeed, the same effect is also encountered in the optimum bus stop design problem: Fewer bus stops lead to higher efficiency (Moura et al. 2012; Schöbel et al. 2009; Nalawade et al. 2016), but this comes at the cost of less customer satisfaction since the average distance to cover for a customer to get to their pickup location is increased. Various ondemand ridepooling service providers as well choose to allow pickup and dropoffs only at selected virtual busstops (i.e. bundled stop locations, meant to reduce the number of stops and amount of detour for the ridesharing vehicles) instead of any node in the underlying street network (Pettersson 2019; Racah et al. 2017).
Conclusion and discussion
Here we have tackled the question how network topology affects the feasibility of ride sharing. For this, we have studied the steady states of a onevehicle ridesharing system in a range of simple homogeneous networks as well as real regional street network topologies using both analytical and numerical methods and analyzed the backlog (i.e. the stoplist) in the system depending on its load.
We find in Fig. 4 that, while similar networks also result in similar scaling behaviour of the stoplist length (i.e. ridesharing is almost indistinguishable between the ring and the line or different grids of the same size), there are large differences between networks with different dimensions (i.e. ring (1D), grid (2D) and star (∞D)). Furthermore, there is a substantial impact of the number of nodes N on the ridesharing predisposition.
We find these differences to be summarized by the cumulative route volume parameter α.
In the light of these findings we have compared a range of very different regional street networks ranging from urban centers to smaller towns and rural areas. Assigning them the same request density and selecting bounding boxes, such that the networks have comparable distances and node numbers N, we find, that ridesharing is topologically harder in urban areas, as their gridlike structure causes routes to be more likely to be distinct while the loose mesh, characteristic of rural areas topologically forces ridesharing on long stretches of the connecting streets. We expect this effect to be even stronger with more realistic request patterns, which would remain largely homogeneous in urban centers, but be centered around settlements in rural areas, effectively lowering the number of active nodes.
This effect is, however, counteracted by the typically inconveniently low request rates for public service options in rural areas. To make use of the beneficial network structure, it would therefore be necessary to convince more customers of participating in shared flexible transport options.
The combination of nearby nodes (coarse graining) may improve ridesharing and thus deliver convenient, efficient public transport. This may be realized in practice by offering the customers a choice of virtual busstops (Pettersson 2019; Racah et al. 2017). Cities on the other hand already have an inexpensive and efficient public transport in lineservices. Further research is needed to determine how introducing lineservices on the most frequented routes would affect or be combined with ondemand ridesharing.
Outlook
As mentioned above, there seems to be a relationship between ridesharing efficiency and network dimension. In future works it would be highly valuable to further explore this connection and establish a network measure, that predicts the slope directly from the network topology.
In order to directly apply the results of this research to realistic ridesharing scenarios, the simplified demand pattern would need to be replaced by more realistic input. Equally one would likely work with several finite sized vehicles that allow small detours, rather than one infinitely large one making zero detour. In the choice of those parameters, intuitions gained from this analysis can be used to make educated guesses for such parameters.
One question in particular to be addressed with such a more detailed model would be an analysis of the gains to be achieved through virtual bus stops in a trade off between efficiency and personal comfort.
Appendix A: Stoplist volume and insertion into a stoplist
We want to know what is the expected (i.e. ensemble average) stoplist volume 〈V〉(n), when n stops are added to an initially empty stoplist according to the procedure described in “Model” section. The route volume V of a stoplist is the number of nodes that can be reached without taking any detour while traversing it. Given that a pickup point is inserted into the list, V_{rest} is the volume of the rest of the route after the pickup position. This is illustrated in Fig. 7 in Appendix A.
Analytical stoplist volume for rings
Consider an N node ring with nodes labelled 1,2,⋯,N. Let a stoplist of length n be constructed as per “Model” section and V_{n} be its volume. Our goal is to find out the expectation value E[V_{n}]:=〈V〉(n).
We go to the continuum limit here for ease of analytical computations (i.e. we approximate the N node ring with a continuous ring of length N). Further, we do our calculations in phase variables angular volumew_{n}∈[0,2π], instead of in V_{n}, by defining
We further assume without loss of generality that the stoplist (with angular volume w_{n}) has its extremities at 0 and w_{n}, effectively going to a rotated coordinate system.
We approach the problem by looking at the expectation value of the increment in volumes Δ_{n}(w_{n},ζ_{n},ζ_{n+1}) when a new stop at ζ_{n+1} is added to an nlength stoplist with angular volume w_{n} whose last stop is at ζ_{n}:
If we know E[Δ_{n}w_{n}], a recursive formula can be derived for E[w_{n}].
The stoplistgeneration process described in “Model” section necessarily means the function Δ_{n}(w_{n},ζ_{n},ζ_{n+1}) will have the form described in Fig. 8.
The main difficulty in computing E[Δ_{n}w_{n}] lies in the fact that although ζ_{n+1} and w_{n} are independent, ζ_{n} and w_{n} are not independent (because ζ_{n}, the last point on the stoplist, influences the value of w_{n}). We therefore make out first simplifying assumption: Ignore this dependence and assume that ζ_{n} is uniformly randomly distributed in the interval (0,w_{n}), i.e. the last point in the stoplist is equally likely to be found anywhere within the volume. Then we have
leading to
Plugging in the piecewiselinear functional form of Δ_{n} as described in Fig. 8 into Eq. (18) yields
and using (15), we get
Now we can go back to the discrete V_{n} from the continuous w_{n} by using Eq. 13:
Pickup location in a stoplist
We see in Fig. 2a and b that V_{rest} computed based on the assumption that the pickup is equally likely to be inserted after any stop in the stoplist, is slightly overestimating the actual value. In reality, however, the pickup location is slightly more likely to be in the second half of the stoplist, as demonstrated in Fig. 9 in the case of the 100 node ring.
Appendix B: Coarse graining street network and homogenizing the edge lengths
Obtaining openStreetMap data
We developed our analytical results of “Analytics” section for unweighted graphs. In “Results” section, we applied the analytical insights to real world street networks, in addition to simple motif networks (Fig. 4). For this purpose, we used freely available and open data from OpenStreetMap project (OpenStreetMap contributors 2020). We used the open source python library OSMnx (Boeing 2017) to download OpenStreetMap networks for a number of geographical regions including Berlin, Göttingen and the Harz national park in Germany. In each case, we downloaded networks including all streets upto ‘secondary’ level (OpenStreetMap Wiki Contributors 2020).
Coarse graining
Since the networks so obtained were still quite large, we coarse grained them by merging any two nodes that were closer than 100 meters into a single node. This is illustrated in Fig. 10, panels (a)  (c).
Converting to unweighted networks
At this stage we had a much smaller network, but the edges were uneven in length, especially between the different regions, making them not comparable in travel times, especially as our ridesharing algorithm is based on unweighted networks. In order to get a representation of the network in the form of an unweighted network that still preserves the topological features, we introduced intermediary nodes in each of the edges so that the target edge length would be 400 meters. So, if an edge (u,v) had a length of 1000 meters, now two nodes w_{1} and w_{2} would be inserted between u and v so that the edges (u,w_{1}),(w_{1},w_{2}) and (w_{2},v) have lengths 400,400 and 200 meters respectively. This step is illustrated in Fig. 10, panels (c)  (d).
Properties of the coarse grained network
The resulting network so obtained (called the “coarse grained network”) has similar average shortest path lengths as the original network. In case of Berlin, we find that the average shortest path length is 5859.6 meters for the original network and 5143.4 meters for the coarse grained one.
In Fig. 11, we demonstrate the difference in the distributions of the edge lengths between the original and the coarse grained network. The original network has a wide spread of edge lengths, as well as large number of very short edges. In the coarse grained network, most of the short edges are gone, and the vast majority of the edges have the desired target edge length of 400 (see Converting to unweighted networks). At the same time Fig. 11b demonstrates that distances between nodes are preserved by the coarse graining procedure.
Availability of data and materials
All data generated or analysed during this study are included in this published article [and its supplementary information files].
Abbreviations
 DARP:

Dynamic dialaride problems
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Acknowledgments
We thank Stephan Herminghaus, Marc Timme, Malte Schröder, Phillip Marszal, Nils Bayer and Felix Jung for fruitful discussions.
Funding
This research was supported by the European Fond for Regional Development (EFRE) through the state of Lower Saxony, and the Max Planck Society.
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DM and NM conceived, designed and carried out the research, DM carried out the simulations, DM and NM wrote the manuscript. Both authors read and approved the final manuscript.
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Manik, D., Molkenthin, N. Topology dependence of ondemand ridesharing. Appl Netw Sci 5, 49 (2020). https://doi.org/10.1007/s41109020002902
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