Detecting electromagnetic signal sources from mobile devices travelling along paths and applications thereof Field of the invention The present invention relates to the field of categorising the location of electromagnetic signal sources close to roads. Background to the invention It is known to estimate the location of electromagnetic signal sources and to use subsequent measurements of signals from the electromagnetic signal sources in location related services, such as estimating the position of a mobile electronic device comprising an electromagnetic signal source receiver. Electromagnetic signal sources used in the methods of the invention are typically terrestrial radio signal sources, such as wireless access points (WAPs), radio- freguency beacons, such as those according to the Bluetooth Low Energy (BLE) specification, which currently broadcast UHF radio waves in the 2.4 to 2.485HGz frequency range, including iBeacons and Eddystone beacons. (Bluetooth, iBeacon and Eddystone are trade marks). However the electromagnetic signal sources may be optical signal sources, for example visible light communications transmitters or transceivers (e.g. Li-Fi transmitters or transceivers). It is known to estimate the position of electromagnetic signal sources from a vehicle driving down the road, while tracking the position of the vehicle using a global navigation satellite system (GNSS) and this is known in the art as war-driving. It is known to implement a positioning system in which mobile devices make measurements of electromagnetic signals from electromagnetic signal sources, a with reference to a database of positioning data concerning signal sources and in some embodiments, expected signals from those sources to thereby enable an estimate of the position of mobile devices to be made. The position estimate may be made locally on the mobile device, or by a server, or a combination thereof. This positioning data is maintained in the form of a database concerning signal sources distributed across a wide geographic area. Measurements of signals from signal sources, or data derived therefrom, are fed back to a central positioning controller which can use that data to update the at least one database, for example adding or removing signal sources. This approach to maintaining a positioning system database, using measurements from individual mobile devices, has been referred to as crowd sourcing. Furthermore, WO 201 1/077166 (Arslan et al.) discloses an arrangement in which initial estimates of the position of signal sources are improved using subsequent measurements of signals from those sources by individual mobile devices and which is especially useful for applications such as indoor positioning where satellite positioning systems are inaccurate or non-functional. It is known to deliberately place electromagnetic signal sources (beacons) at specific known locations to facilitate position detection and optionally to trigger specific actions when they are detected by mobile devices. For example, an application executed on a mobile device might cause an advertisement to be displayed or another action to be carried out when the mobile device detects signals from a specific electromagnetic signal source and so it can be inferred that the mobile device is proximate to that signal source. Methods of maintaining (e.g. creating, building or editing) a database of positioning data and/or using positioning data to estimate the position of a mobile device are disclosed in US 8634359, US 2014/0243025, US 2015/01 19071 , US 2014/0243015, US 2014/0365488, US 2015/0189467, US 2015/0195810, US 2015/0172872, US 2015/0309183, US 2014/0379476, US 15/014,393, PCT/GB2015/052799, and PCT/GB2015/052798, which are incorporated herein by virtue of this reference. The invention aims to address problems with existing positioning systems. Some aspects of the invention aim to facilitate estimating on what side of a road or roadway a user is located, for example to route a vehicle to them to pick them up. Summary of the invention According to a first aspect of the invention there is provided a method of maintaining (e.g. creating, building or editing) a database of positioning data, the positioning data specifying location data concerning each of a plurality of electromagnetic signal sources or signals from electromagnetic signal sources, the method comprising determining that a mobile device is travelling along a path, the mobile device comprising at least one detector for signals from the electromagnetic signal sources, the method comprising detecting a plurality of electromagnetic signal sources at a plurality of locations along the path and storing parameters indicative of the variation in the strength of signals from the electromagnetic signal sources along the path and/or adjacent the path (e.g. where the path is on a road, along individual sides of the road). The parameters may be stored, thereby providing stored data concerning the strength of signals from signal sources along the path and/or along individual sides of the road. The path and/or the path of individual sides of the road may also be stored. The stored data may comprise measurements of the strength of signals from individual signal sources (typically with reference to their MAC address or another identifier) at locations along the path and/or along individual sides of the road or parameters of equations which describe the variation in the strength of signals from individual signal sources at locations along the path and/or along individual sides of the road. This assists, because the resulting data concerning the strength of signals from signal sources along the path and/or along individual sides of the road improves later estimates of the position of mobile user devices located along the path and/or along individual sides of the road (e.g. on sidewalks adjacent the road). According to a second aspect of the invention there is provided a method of maintaining (e.g. creating, building or editing) a database of positioning data, the positioning data specifying location data concerning each of a plurality of electromagnetic signal sources or signals from electromagnetic signal sources, the method comprising determining that a mobile device is travelling along a path, the mobile device comprising at least one detector for signals from the electromagnetic signal sources, the method comprising detecting a plurality of electromagnetic signal sources and determining on which side of the path of the mobile device each of the electromagnetic signal sources are located. The mobile device is typically a passenger vehicle, such as a car. The mobile device may however be a mobile user device, such as a mobile telephone, personal electronic device, such as a tablet or laptop computer, or a wearable electronic device such as a garment, watch, armband, glasses, contact lenses, in which case it proceeds along the path by being carried in a passenger vehicle or by a human person. It may be that the mobile device is caused to travel along the path, for example, instructed to travel along a specific route. It may be that the mobile device travels along the path outside of the control of the methods and system of the invention but that it is detected that the mobile device is travelling along the path. Accordingly, the invention allocates electromagnetic signal sources to one side or another of a path. The at least one detector of the mobile device is able to determine on which side of the path the electromagnetic signals are located. For example, the at least one detector may comprise a plurality of detectors for the electromagnetic signal sources which are spaced apart across the width (i.e. direction perpendicular to the direction of travel) of the mobile device. The distance from each detector to the respective electromagnetic signal sources can be estimated and the difference between these distances indicates on which side of the path of the mobile device the electromagnetic signal source is located. It may be that the at least one detector is directional and is capable of determining the direction in which electromagnetic signals are received. The at least one detector may comprise a signal blocking or reflecting element to cause the at least one detector to be directional. According to a third aspect of the invention there is provided a method of maintaining (e.g. creating, building or editing) a database of positioning data, the positioning data specifying location data concerning each of a plurality of electromagnetic signal sources or signals from electromagnetic signal sources, the method comprising determining that a mobile device is travelling along a path, the mobile device comprising at least one detector for signals from the electromagnetic signal sources, the method comprising detecting a plurality of electromagnetic signal sources at a plurality of locations along the path and storing parameters indicative of the variation in the strength of signals from the electromagnetic signal sources along the path and/or adjacent the path (e.g. along individual sides of the road where the path is on a road), determining on which side of the path of the mobile device each of the electromagnetic signal sources are located and processing pre-existing estimates of the location of the electromagnetic signal sources or the strength of signals from the electromagnetic signal sources at locations, and the determined side of the path of the mobile device on which each electromagnetic signal source is located (and typically also the path itself) to generate or improve estimates of the position of the electromagnetic signal sources. Thus, the measurements obtained while moving along the path can be used to generate or improve estimates of the position of electromagnetic signal sources. For example, it may be that estimates of the position of electromagnetic signal sources exist in a region but potentially with a large systematic error, e.g. because the position estimates are based on relatively low numbers of measurements of signals from the signal sources or because global navigation satellite services (GNSS) are or poor accuracy or are not available in the region. This can facilitate subsequent position estimates. Signal sources may be portioned into those on one side of the path and those on the other. According to a fourth aspect of the invention there is provided a method of maintaining (e.g. creating, building or editing) a database of positioning data, the positioning data specifying location data concerning each of a plurality of electromagnetic signal sources or signals from electromagnetic signal sources, the method comprising determining that a mobile device is travelling along a path, the mobile device comprising at least one detector for signals from the electromagnetic signal sources, the method comprising detecting a plurality of electromagnetic signal sources at a plurality of locations along the path and processing estimates of the location of each of the plurality of electromagnetic signal source and measurements by the detector of signals from the electromagnetic signal sources to improve estimates of the location of each of the plurality of electromagnetic signal sources. Thus, the measurements obtained while moving along the path can be used to improve estimates of the position of electromagnetic signal sources, for example to smooth out the spacing electromagnetic signal sources along the path or to correct the spacing of the electromagnetic signal sources in a direction parallel to the path. The method may comprise assuming that the mobile device is moving at a constant or smoothly changing speed along the path. Thus measurements of signals from electromagnetic signal sources at spaced apart time (e.g. a time series) can be assumed to have been made while the mobile device is at locations, the distance between each of which is more accurate than would be the case is a measurement of the position of the mobile device when each measurement of signals from electromagnetic signal sources was taken independently each time that such measurements were made. It may be that the mobile device is, or is located within, a vehicle and the path is a road or a roadway or lane of a road. It is especially useful to determine on which side of a road, or roadway (either or which may comprise one or more lanes) an electromagnetic signal source is located. For example, if it is known on which side of a road, or roadway, an electromagnetic signal source is located then it is possible to route a vehicle (such as a taxi, or autonomous car) to a person who is determined to be close to that electromagnetic signal source (typically because they have a mobile user device comprising an electromagnetic signal source receiver, such as a radio receiver, and it is determined that they can detect the specific electromagnetic signal source or are within a particular distance of the specific electromagnetic signal source). The method may comprise estimating the position, in two or three dimensions, of an electromagnetic signal source which is detected at a plurality of locations along the path, taking into account path data which specifies the path. It can assist to estimate the position of an electromagnetic signal source to determine that the position of the mobile device which detects the electromagnetic signal source is on a specific path. Determining the path along which the mobile device is travelling may take into account data measured by one or more sensors of the mobile device, for example one or more sensors selected from a group consisting of: an accelerometer, a magnetometer, a gyroscope, a speedometer. It may be that the mobile device is, or is located in, a vehicle and the path is the path of a road, or roadway or a lane thereof and the method comprise the step of determining on which road, or roadway or lane thereof, the mobile device, or the vehicle in which it is located, is travelling. The method may comprise estimating the position of the mobile device, querying geographical data (stored in a geographical data database) specifying the path of a plurality of roads, roadways or lanes thereof, and thereby determining which road, roadway or lane thereof the path of the mobile device is following. The method may further comprise determining the direction of movement of the mobile device along the road, roadway or lane thereof. The method may comprise the step of estimating the speed of movement of the mobile device along the path. By estimating the speed of movement of the mobile device along the path, it is possible to estimate the position of the mobile device at given points in time while it is moving along the path more consistently and therefore to make more consistent estimates of the position of the electromagnetic signal sources than would be the case if estimates of the position of the mobile device at different points in time were made independently. It may be that the method comprises the step of estimating the position of the mobile device using a global navigation satellite system receiver of the mobile device before and after the path of the mobile device passes through an underground region where measurements of position using the global navigation satellite system receiver are of low accuracy or not available. The method may comprise estimating whether each electromagnetic signal source is indoors or outdoors and storing that estimate. It is possible to estimate whether an electromagnetic signal source is indoors or outdoors from the distribution of the strength of signals from the electromagnetic signal source along the path. The method may comprise estimating whether each electromagnetic signal source is or is not located between a road facing wall of a building and the road, or estimating whether each electromagnetic signal source is located on or above a sidewalk. It is possible to estimate whether an electromagnetic signal source is or is not located between a road facing wall of a building and the road, or estimating whether each electromagnetic signal source is located on or above a sidewalk from the distribution of the strength of signals from the electromagnetic signal source along the path. It may be that the path is the path of a road, roadway or lane thereof, the method further comprising determining a location for a pick up of a user by a passenger vehicle, the user having a mobile electronic device having an electromagnetic signal receiver, the method comprising detecting electromagnetic signals from an electromagnetic signal source at the electromagnetic signal receiver, determining on which side of the road or roadway the mobile electronic device is currently located and determining a location for a pick up of a user by a passenger vehicle on the respective side of the road or roadway. The method may further comprise the step of calculating a route for the passenger vehicle to follow to pick up the user at the pick up location, which route takes into account the determined side of the road or roadway. Thus, the method may comprise selecting a route from amongst a plurality of possible routes in which the passenger vehicle travels along the road to the pick up location on the correct side of the road and/or in the correct roadway to be adjacent the user. The passenger vehicles are motorised vehicles capable of carrying one or more people, having at least three, and typically at least four ground engaging wheels. They may be automobiles with internal combustion engines, electrical motors and batteries, or hybrid engines, and may have a human driver or be autonomous vehicles which comprise a controller which controls the motion of ground engaging wheels to direct the vehicle along a route without continuous human input and a plurality of sensors to guide the vehicle and avoid obstacles such as other vehicles, pedestrians and so forth. Autonomous vehicles and their operation are well described in the art, for example in US 2016129908 (Volvo Car Corporation). The passenger vehicle have electronic devices comprising radio receivers to receive instructions concerning the location of a pick up and these may be integral to the passenger vehicle, for example radio receivers of a wireless communications interface of an autonomous vehicle, or mobile electronic devices, such as mobile telephones, which are not integral to the vehicle but are simply carried within the vehicle, for example demountably fixed to a stand or holder to enable a human driver to view information which they display. The method may further comprise the step of selecting a passenger vehicle to pick up a user from amongst a plurality of passenger vehicles, comprising calculating a (shortest in time or distance) route for each of the plurality of passenger vehicles to follow to pick up the user at their current location, which route takes into account the estimate of whether the current position of the mobile user device is on the first or second side of the road, or the side of road of the estimated geographical location of the current position of the mobile user device, and selecting the passenger vehicle for which the calculated route is shortest (in time or distance) to pick up the user. The optional features mentioned above are optional features of each aspect of the invention. The methods of the invention may be carried out by one or more hardware processors. The one or more hardware processors typically execute computer program instruction stored on one or more hardware data stores. Description of the Drawings An example embodiment of the present invention will now be illustrated with reference to the following Figures in which: Figure 1 is a schematic diagram of a computer system; Figure 2 is plan view of a vehicle proceeding along a lane of a road with electromagnetic signal sources on either side; Figure 3 is a plan view of a vehicle proceeding along a lane of a road, through a tunnel, with electromagnetic signal sources on either side; Figure 4 is a flow chart for carrying out the methods of the present invention; Figure 5 is a schematic diagram of a road network; and Figure 6 is a graph representation of a road network. Detailed Description of an Example Embodiment Figure 1 is a schematic diagram of a computer system 1 having a positioning system server 2, a path server 20, a plurality of mobile user devices 100 and a plurality of passenger vehicles 150, each of which may communicate through the internet 40 and wireless communication services 42 (such as 3G, 4G, 4.5G, 5G wireless telecommunications networks, WiFi, WiMax and so forth). The positioning system server 2 comprises at least one processor 4 which is in electronic communication with solid state memory 6 which stores computer program code which when executed on the at least one processor causes the processor to execute the server functionality described below, and a positioning system database 8, stored on computer readable tangible storage means (such as solid state memory, hard discs, optical discs etc.) which stores positioning data 10 and geographical data 12. The positioning data concerns a plurality of terrestrial electromagnetic signal sources (such as wireless access points, WAPs, radio beacons, such as those according to the Bluetooth Low Energy (BLE) protocol etc.) For each signal source it specifies the MAC address of the signal source and optionally properties of the plurality of electromagnetic signal sources, such as the type of the electromagnetic signal sources or transmit signal strength. According to the invention, the positioning data specifies on which side of a specific road the signal source is located. This data may take the form of a reference to a road in a database of road paths and a reference to a side of that road. The side of the road may be referred to in absolute terms (e.g. north, south, east, west) or relative to a direction, for example to the path of the road specified in a vector format. This data enables a later estimate of the side of the road on which a user having a mobile user device which can detect the signal source is located. The positioning data may further comprise estimate geographical locations (e.g. 2D or 3D coordinates, for example latitude, longitude and optionally altitude) of the electromagnetic signal sources. Such positioning data enables a later estimate of the current position of a mobile user device from measurement of signals (e.g. the strength of signals, or time of receipt of signals) from the electromagnetic signal sources by a radio (or other electromagnetic signal) receiver 22 of the mobile user device, and the estimated positions of electromagnetic signals sources as stored in the positioning data. The path server 20 comprises at least one processor 22 which is in electronic communication with solid state memory 24 which stores computer program code which when executed on the at least one processor causes the processor to execute the server functionality described below, and a path database 26, stored on computer readable tangible storage means (such as solid state memory, hard discs, optical discs etc.) which stores data concerning paths along which mobile device (such as mobile user devices and vehicles) may travel. The data concerning paths typically comprises, for each of a plurality of roads, data specifying the path followed by the road. This may comprise data specifying the path of the centre of the road, or roadways or lanes which make up the road, or of the boundaries on either side of the road of roadways which make up the road. The data may specify whether a road or roadway has a sidewalk and/or is bounded by a building (e.g. a wall of above a defined height) at particular locations along the length. One skilled in the art will appreciate that the positioning system and path servers may be combined into one server, or implemented separately on one or more physical server machines, distributed, virtualised and so forth. The mobile user devices 100 comprise radio signal transceivers 102. Typically, some or all of the mobile user devices are cellular telephones comprising one or more processors 104 in electronic communication with solid state memory 106 which stores program code which cases the devices to carry out the procedure described below. Typically, some of the mobile user devices execute the Android operating system (for example Android 6.0.1 , or later) (Android is a trade mark of Google, Inc.) and/or the iOS operating system (for example iOS 9.2 or later) (iOS is a trade mark of Apple, Inc.) Some or all of the mobile user devices comprise satellite positioning system modules 108, which receive signals from orbital satellites during operation and process these signals to estimate the position of the mobile user devices. Some or all of the mobile devices comprise one or more additional sensors 1 10, data from which is used to estimate the position of the mobile user device. The additional sensors may be selected from a group consisting of: an accelerometer, a gyroscope, a step counter, a camera. Data from the one or more said sensors may be processed to improve the estimated position of the mobile device and/or to estimate the path of the mobile user device. The mobile user devices have user interface peripherals 1 12, for example a touch screen, enabling a user to input data (for example to request a pick up by a passenger vehicle) and to view information, such as a map or information about a forthcoming pick up by a passenger vehicle (such as the expected arrival time of a passenger vehicle). User interface peripherals 112 may also comprise a microphone to receive verbal instructions, a loudspeaker to give audible instructions, an accelerometer to receive gesture based instructions, and so forth. One or more of the mobile user devices may be wearable components, for example watches, glasses, shoes, armbands or contact lenses. The mobile user device may comprise a plurality of separate or separable components which are in (typically direct) wired or wireless communication with each other (e.g. a mobile telephone, tablet or computer and a separate or separable wearable component). Typically, it is not necessary to distinguish between the position of individual components if they are typically on the person of the user as the aim is to establish the position of the user. The passenger vehicles are motorised vehicles capable of carrying one or more people, having at least three, and typically at least four ground engaging wheels. They may be automobiles with internal combustion engines, electrical motors and batteries, or hybrid engines, and may have a human driver or be autonomous vehicles which comprise a controller which controls the motion of ground engaging wheels to direct the vehicle along a route without continuous human input and a plurality of sensors to guide the vehicle and avoid obstacles such as other vehicles, pedestrians and so forth. Autonomous vehicles and their operation are well described in the art, for example in US 2016129908 (Volvo Car Corporation). In a first example, a passenger vehicle 150 passes along a route 170 on a road 180. The road has two roadways 182 and 184 which run in opposite directions. The road has a first side 190 and an opposite second side 192. Sidewalks 186, 188 are located on the first and second sides of the road. On either side of the road there are located a number of radio beacons 194, such as a WAPs and radio beacons according to the Bluetooth Low Energy (BLE) specification. Some of these are located in the open, for example, on or above a sidewalk (e.g. on the external face of a building, overlooking the road) and others are located in buildings 196. The passenger vehicle has first and second radio receivers 152, 154 located on the left and right sides of the vehicle, capable of detecting the radio beacons 194. The first and second radio receivers measure the strength of signals from radio beacons as the vehicle passes along the path and the difference between these strengths can be used to determine on which side of the vehicle, and therefore on which side of the road, the individual signal sources are located. In alternative embodiments, there may be a single radio receiver which is directional (for example has a reflector or attenuator configured to receive electromagnetic signals preferentially in a specific direction). The passenger vehicle also has sensors comprising a GNSS receiver module 158 (e.g. a GPS receiver), an accelerometer 160, a gyroscope 162 and a magnetometer 164. These can be used to estimate the location, direction and speed of travel of the vehicle. In some embodiments, instead of using sensors which are integral to the vehicle, the sensor measurements are carried out entirely by a mobile user device 100 which is located in the vehicle, for example about the person of a passenger of the vehicle, or demountably fixed in the vehicle, for example to a dashboard of the vehicle. With reference to Figure 4, a stream of measurement data is received from the sensors 50 and stored for subsequent analysis. The received data may include: - Estimates of the position of the vehicle from the GNSS receiver

- A measure of the accuracy of the estimates of the position of the vehicle, which is also output by typical GNSS receivers

- Measurements of signals received from radio beacons from both sensors 152 and 154, for example their MAC code or other identifier (which is typically broadcast by radio beacons) and the strength of signals received from the radio beacons.

- Measurements of acceleration of the vehicle from the accelerometer

- Measurements of the orientation of the vehicle from the magnetometer - Measurement of the orientation or changes in orientation of the vehicle from the gyroscope. In some embodiments estimates of the position are also derived from the measurement of signals received from radio beacons, with reference to positioning data specifying the location of those radio beacons, where available. The received estimates of the position of the vehicle are used to query the database of route data and therefore to determine 52 that the vehicle is on a specific road and heading in a specific direction. Where there is sufficient data available, it may be determined that the vehicle is in a specific roadway or even a specific lane. Thus, the path along which the vehicle is moving can be determined as the path of that road, roadway or lane. The measurement data is analysed at different time steps to estimate 54 the progress of the vehicle along the path formed by the road, roadway or lane. Position estimates from the GNSS module and acceleration data from the accelerometer are especially useful to determine where the vehicle is along the path at a given time. Data from the first and second radio receivers (152, 154) (or a directional radio receiver) is used to determine on which side of the road individual signal sources are located. This is stored in the database of positioning data. Still further, measurements of the strength of signals from individual signal sources are stored as the vehicle progresses along the path. These measurements are stored, typically separated by side of the road. They are typically stored with reference to the path. This data is useful for later estimates of position to estimate the position of mobile user devices which later detect the signal sources, and to facilitate further or improved estimates of the position of the signal sources by estimating the position of mobile user device which later detect the signal sources and processing the strength of the detected signals to generate further or improved estimates of the position of the signal sources. In some embodiments, that data which is stored takes the form of parameters of equations (e.g polynomials or wavelets) which best fit the measurements of the strengths of the signals along the path. These can be stored with reference to the path to later estimate the location along the path of mobile user devices which detect signals from the same signal sources. Furthermore, it is estimated 58 whether individual radio beacons are indoors or outdoors from the variation in the strength of signals from those radio beacons along the path of the vehicle. Signals from radio beacons which are indoors will be more attenuated and may vary in a way which is significantly different from the attenuation of radio signals in air. However, radio beacons which are outdoors, on surfaces facing the road, will typically be detectable from a relatively wide range of orientations and the strength of signals from them will typically vary along the path of travel in a way which is consistent with the typical attenuation of radio signals in air. Where it can be determined that a radio beacon is Iocated on or above a sidewalk, from the distribution of signals around the radio beacon, it may also be stored in the database of positioning data that the radio beacon is Iocated on a sidewalk. Estimates are typically also made of the position of the radio beacons 62, using triangulation given estimates of the distance to the respective radio beacons at different points along the path and this may also be stored in the database of positioning data or used to improve estimates of the position of the respective radio beacons. In a further example shown in Figure 5, the path of a vehicle is along a lane of a road which extends through a tunnel 198 (which has a sidewalk within the tunnel on one side but not the other). This means that the estimates of the position of the vehicle by the GNSS receiver Iocated in the vehicle (integral to the vehicle or in a mobile user device Iocated within the vehicle) are not available for part of the path. Nevertheless, it remains possible to estimate the side of the road of the radio beacons within the tunnel using the path of the road. It is also possible to estimate the path of the vehicle (where reliable GNSS signals are not available) by making measurements of the position of the vehicle where available and using measurements form the sensors on the vehicle, notably the accelerometer and magnetometer and/or gyroscope, to infer the path of the vehicle between a location before the tunnel or other region where an accurate estimate of the position of the vehicle using a GNSS receiver is not available to another location after that region. Indeed, as vehicles generally change course only gradually their path can usually be estimated relatively accurately by this method and this can provide a good basis for measuring the position of signal sources located in regions where GNSS measurements of position are not available or are of poor accuracy. As well as covered regions, this may apply in built up areas, for example between high buildings. As the side of the road on which individual electromagnetic signal sources are located has been identified, it is possible to estimate on which side of a road a mobile user device (and therefore a user) is located by using a radio receiver (or other electromagnetic signal detector) of the mobile user device to detect signal sources. It is then assumed that the user is located on the side of the road of the signal source which is closest to the mobile user device. The closest signal source may be determined as the signal source from which the strongest signal is received, or from which the least attenuated signal is received (which requires knowledge of the transmit power of the signal source) or in some embodiments, which is determined to be closest from round trip time calculations or other signal timing based distance measurement techniques. It is helpful to determine on which side of a road a mobile user device (and therefore a user) is located for a number reasons. Firstly, this can assist with estimating the location of the user as it would be unusual for a user to be standing in a roadway. Secondly, the side of the road on which the mobile user device (and therefore the user) is located is used to route a passenger vehicle to the user. The passenger vehicle may be a taxi (with a human driver) or autonomous vehicle, which may be directed to pick up the user on the side of the road on which they are currently located. It is helpful to determine the correct side of the road for a user to be picked up because this can avoid a requirement for the vehicle to make a turn in the road, or the user to cross the road, and can enable the vehicle to be more efficiently routed to the user. Figure 5 is a schematic diagram of a road network, comprising a plurality of two way roads 200A through 200F, which join at various intersections 250A through 250D. Assuming that this represents a country in which vehicles drive on the right hand side of the road (in the direction of travel), If a user is found to be on the side of the road 200B shown with the letter A when they request a pick up by a vehicle 150, it would be best for the vehicle to turn right at junction 250D and left at junction 250B to be on the correct side of the road to pick up the user. If however the user is found to be on the side of the road 200B shown with the letter A when they request a pick up by a vehicle, it would be best for the vehicle to turn left at junction 250D and right at junction 250A, to be on the correct side of the road to pick up a user. Thus, a vehicle can be routed to the user for a pick up in dependence on the determined side of the road. In order to enable routing, the road network of Figure 5 can be represented in electronic form in a graph data structure shown in Figure 6, with nodes 300A through 300G, coinciding with junctions and bends, and data specifying which nodes are connected with roadways 302 and in which directions traffic may pass (e.g. a single direction 302, or bidirectional 304). More sophisticated data structures representing road networks, known in the art, include data concerning the path of individual roadways and lanes, the speed limit of road sections between nodes, measured average, current or predicted speeds of traffic on road sections between nodes, the location of speed cameras, traffic lights and so forth. Routing can be carried out by executing known algorithms for finding paths through graphs, for example Dijkstra's algorithm (Dijkstra, E. W. (1959). "A note on two problems in connexion with graphs" (PDF). Numerische Mathematik 1 : 269-271., and Cormen, Thomas H.; Leiserson, Charles E.; Rivest, Ronald L; Stein, Clifford (2001). "Section 24.3: Dijkstra's algorithm". Introduction to Algorithms (Second ed.). MIT Press and McGraw-Hill. pp. 595-601 , each of which is incorporated herein by virtue of this reference) or the Bellman Ford algorithm, or (to obtain multiple options), the K shortest path routing algorithm (Michael Gunther et al.: "Symbolic calculation of K- shortest paths and related measures with the stochastic process algebra tool CASPA". In: Int'l Workshop on Dynamic Aspects in Dependability Models for Fault- Tolerant Systems (DYADEM-FTS), ACM Press (2010) 13-18, incorporated herein by virtue of this reference). Once a pick up has been requested, a passenger vehicle is allocated to make the pick up (or predetermined, for example if a specific passenger vehicle is allocated to or property of the user), a route for the passenger vehicle to travel to the user to make a pick up on the appropriate side of the road is calculated, and the route is transmitted to the passenger vehicle to enable the vehicle to pick up the user. The position of the passenger vehicle may also be transmitted to a mobile electronic device of the user, for example along with map data, to enable the user to view the approach of the passenger vehicle. The position of the user and in particular their side of the road may be updated and the route changed dynamically while the passenger vehicle is travelling to make a pick up. The estimate of the side of the road on which the mobile user device (and so the user) is located may also be used to select a passenger vehicle from amongst a plurality of passenger vehicles to direct to pick up the user. A vehicle which is closest to the user, or even closest to the centre of the road adjacent the user, may not be the vehicle which can most quickly pick up the user at side of the road on which they are located. In order to implement this, a plurality of vehicles in a region around the current position of the user are identified and estimates are made of the time (or in some embodiments distance) which would be required for the vehicle to travel to a user to pick up the user at the estimated side of the road of their current location. The vehicle which would require the shortest time (or distance) to travel to pick up the user at the respective side of the road is selected to pick up the user and directed to pick them up at their current location on the respective side of the road. Estimates of the position of electromagnetic signal sources obtained by the methods above may be used to later estimate the position of mobile user devices with electromagnetic (typically radio) receivers to detect the signal sources, and these estimates of the position of mobile user devices can be used to improve estimates of the position of the electromagnetic signal sources and/or newly detected electromagnetic signal sources, for example as set out in WO 201 1/0771666 (Arslan et al.) Further, the measurements which are made as a passenger vehicle proceeds along a path are useful to estimate the location of signal sources where there is already data available in the form of existing estimates of the location of those signal sources (e.g. of relatively poor quality) or other stored measurements of signals from the signal sources made by mobile user devices which have already detected those signal sources. For example, it may be that there are regions in which existing estimates of the position of signal sources are of low accuracy and/or contain large systematic errors. However, by determining that a vehicle is proceeding along a path and that signal sources are located to the right or the left of the vehicle as it proceeds along the path, this enables estimates of the position of these signal sources to be corrected to better fit the additional data. Furthermore, it can be assumed that a vehicle will proceed along the path at a constant or smoothly changing speed. Accordingly, by measuring this speed or change of speed using periodic measurements of the speed of the vehicle (and/or accelerometer data) it is possible to accurately estimate the distance between the locations where the vehicle was located when measurements that are spaced apart in time (e.g. in a time series ever 1 , 5 or 10 seconds etc.) This provides an additional constraint enabling improvements in the estimates of the relative spacing between and therefore the position of the electromagnetic signal sources which are detected from the vehicle. As the invention facilitates the estimation of the position of electromagnetic signal sources adjacent a road it also facilitates the estimation of the path of lanes in a road, or roadway (or boundaries therebetween), and the path of centre line or a road or roadway. This can be carried out by estimating the position of subsequent vehicles travelling along a road using measurements of the strength of signals from the signal sources identified in the steps set out above, and the estimates of the position of those signal sources, to determine the paths along which vehicles move in particular directions, and to use this data to determine boundaries between lanes and roadways and also to estimate the position of the centre line of a road or roadway. An estimate of the path of the centre line of a road or roadway can also be obtained by fitting a curve through the estimated positions of electromagnetic signal sources which have been found to be on either side of the road.