Optimisation in Car-Sharing Systems

For many people the costs of owning, running and maintaining a car are simply too big to justify having one. Having to pay hundreds, if not thousands of pounds to cover insurance, road tax and fuel costs for only a few short journeys a week to work or to the shops doesn’t make financial sense. One alternative to this that has proven popular is ‘car-sharing‘ or ‘car-clubs‘. These schemes allow members to access vehicles owned by the company by reserving them in advance so they can use them for the short trips they might make in a day. This could be anything from picking up the kids from school, popping down to the shops or going out for a few hours at the weekend.

There are several other benefits to using a car-sharing schemes in addition to any potential financial savings. By using a single vehicle to do several different people’s short trips means that less cars are on the road in total reducing traffic congestion and vehicle emissions. Even something as simple as being able to guarantee a parking space when you reach your destination can be attractive, particularly in busy inner cities.

Where can operational research help?

There are lots of problems in car-sharing systems where operational research can help. Since there is a clear trade-off between the level of service offered by the company and the cost that the customer needs to pay to use it, ideally companies want to design the best system to maximise their profits.

Long term decisions such as choosing where to put your stations, how many stations you need and how big to make each station all need to be made. These are decisions cannot be easily altered and play a large role in the overall profitability or viability of the system. This means that any evidence that can help find the best outcomes is desirable.

Other decisions that need to be made on a shorter horizon such as that of how the fleet size and its makeup can also benefit. Too many vehicles means that you will be paying for vehicles sitting idly around in parking spaces, too few and you risk missing out on potential journeys and annoy your customers.

Relocating vehicles

One key problem that optimisation can help with is this issue of vehicle relocation. This originates in one-way car-sharing systems due to the fact that people tend to pick up cars in a different place to where they leave them. This is one of the main attractions of the system, but it causes too many vehicles to be left at stations where demand is low and too few vehicles at stations where demand is high. This might be during the morning commute from the train station to office buildings. People who arrive on the 7:50 train might drive all the cars from the station to their offices, leaving no vehicles for people arriving later on the 8:15 train to use.

relocation

Trips can cause vehicles to end up in the wrong places later in the day

As adding extra vehicles into the system costs money a better approach would be to more effectively utilise the vehicles you already have. There are several ways that have been suggested to do this.

  1. Pricing incentives

One clever way of getting vehicles to the right place at the right time is by encouraging the users of the vehicles to do it for you! If flexible pricing policies are implemented then you can make it more desirable for users to make trips to stations where cars are needed and less desirable to make trips to stations with an excess of vehicles. Two approaches suggested by Barth, Todd and Xue are that of trip-sharing and trip-splitting. Their system was based at a campus university in the USA and they found that by encouraging people to share rides to and from campus (trip-sharing) or split groups travelling to and from campus (trip-splitting), they could reduce the imbalance of vehicles built up.

There are of course several problems with this approach. Some journeys aren’t really that flexible for customers to make such as the commute to work and in real life people are often unwilling to share vehicles with complete strangers.

  1. Only allowing profitable trips

The idea behind this approach is that operators don’t need to worry about the distribution of vehicles, if they only allow trips to be made in directions that keep the system balanced in the first place. In theory this is a good idea, but it requires knowledge of every single customers destination. Another drawback is the fact that customers get annoyed if they are told that they can’t make a particular journey and can’t understand why.Room for growth is limited in this system and customer satisfaction will be very low, so it is not realistically practical.

  1. Relocating vehicles

The most realistic way of solving the vehicle imbalance issue in commercial systems is to simply hire a set of workers whose job it is to move vehicles to where they need to be. They can either move between stations by driving a vehicle that they are relocating or they can travel on foot or by bicycle from station to station. This is the most common method used in car sharing schemes around the world.

Relocating vehicles can have a big impact on the profitability of the overall scheme, good relocations save money, bad ones cost money. This means that doing relocations in an optimal way is extremely important; several early schemes failed because they couldn’t maintain the same service levels as they sought to expand.

It is also important to consider relocations when other decisions such as the location of stations are made. Recent works such as Boyaci, Zografos and Geroliminis have considered ways of doing this for real world systems such as Nice in France.

 

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