Shift hours and overtime

A shift has a minimum start time (minStartTime) and a maximum end time (maxEndTime). All travel and visits occur within that time span.

For example, Carl works from 09:00 until 17:00. On Monday 1-feb, create a shift with minStartTime 09:00 and maxEndTime 17:00. The solver assigns three visits on Monday to him. Because of the service duration of those visits and the travel to, from and between those visits, his shift has an endTime of 16:30. That’s well before the maxEndTime of 17:00:

Carl takes PTO on Tuesday 2-feb, so don’t create a vehicle shift for Carl on that day. He also takes PTO the next morning, but works in the afternoon: Create a shift on Wednesday 3-feb with minStartTime 13:00 and maxEndTime 17:00. The solver assigns him 2 visits on Wednesday and his shift ends at 17:00, just in the nick of time.

A shift starts on the minStartTime, except in some rare cases for which it starts later (see below). A shift must end before the maxEndTime.

What happens if Carl works too long and overtime is forbidden?

For example, Carl’s assigned visits force him to work until 18:00, but his shift has a maxEndTime of 17:00. This is an infeasible schedule. The model penalizes the amount of time that Carl finishes his shift too late as a hard constraint: one hour. This automatically incentivises the solver to assign other or fewer visits, potentially leaving some visits unassigned.

However, if overtime is allowed, Carl can work longer, but it is undesirable. Still, he can’t work forever. His shift must end before the next shift starts, with enough time for Carl to sleep and eat in between. To allow for overtime, use maxSoftEndTime for the normal end of his shift and maxEndTime for the worst possible end of his shift.

For example, Carl can work two hours of overtime: create a shift with a maxSoftEndTime of 17:00 and a maxEndTime of 19:00. The model penalizes the amount of overtime as a soft constraint: one hour. This automatically incentivises the solver to assign some of his visits to other matching drivers without overtime, if possible. But if push comes to shove, the model incurs as much overtime as needed to avoid leaving visits unassigned.

Sometimes a little bit of overtime can go a long way. For example, given two visits on an island reachable by ferry:

By incurring one hour of overtime, a single driver can service both visits on the same day. If overtime is forbidden, the solver has no other option than to assign the two visits to separate shifts. That reduces productivity dramatically, because the travel time by ferry has to happen twice as much.

In some cases, the employer doesn’t have to pay for the travel to the first visit. Instead, it comes out of the employee’s personal time, regardless if the employee lives next near the first visit, or the other side of the region.

To disregard the travel time to the first visit, use minFirstVisitArrivalTime instead of minStartTime. For example, Carl needs to arrive at his first visit at 9:00. Create a shift with a minFirstVisitArrivalTime of 9:00. Because it takes him an hour to drive there, Carl’s shift actually starts at 8:00 today. Tomorrow, his first visit might be around the corner and his shift start at 8:55 to arrive there at 9:00 too.

Using only minFirstVisitArrivalTime is dangerous. There is a limit on Carl’s willingness to work for free. But the solver is incentivised to maximize use of that free travel time, to fit more visits during the normal shift hours.

For example, if there’s a visit on an island with a long travel time by ferry, the solver assigns that travel during Carl’s personal time. But to arrive at the island visit at 9:00, Carl has to get up at 5:00 to depart at 6:00. To prevent that, set a minStartTime of 7:00. This way, Carl departs at 7:00 to arrive at the island visit at 10:00. It’s a compromise: 2 hours of Carl’s personal time and 1 hour of the employer’s time.

Similarly, the employer doesn’t always have to pay to travel back home from the last visit.

To disregard the travel time from the last visit, use maxLastVisitDepartureTime instead of maxEndTime. For example, Carl must finish his last visit at 17:00, regardless of how much time it takes him to get home. Create a shift with a maxLastVisitDepartureTime of 17:00.

Again, using only maxLastVisitDepartureTime is dangerous. For example, Carl finishes his last visit at 17:00, but it takes 3 hours to get home at 20:00. To prevent that, set a maxEndTime of 19:00, to ensure Carl is always home in time for dinner. Regardless of where his last visit is located.