What Is Interlining? When One Bus Becomes Another Route

A passenger reaches the end of a route and notices that the destination sign changes.

The bus that was Route 40 is now Route 50.

It may even continue with some of the same passengers aboard.

This is an example of interlining.

What Is Interlining?

Interlining occurs when the same vehicle operates trips on more than one route as part of the same block.

For example:

• Route 40 outbound
• Route 40 inbound
• Route 50 outbound
• Route 50 inbound

The bus does not necessarily return to the garage between routes.

Why Interline?

One reason is efficiency.

Demand is rarely identical on every route throughout the day.

A bus may be needed on one route during the morning but better used somewhere else later.

Interlining can help an agency:

• Reduce unnecessary idle time
• Reduce deadhead
• Use vehicles more efficiently
• Create through-service opportunities
• Match resources to changing demand

Through-Riding

Sometimes interlining allows passengers to stay on the same bus while the route number changes.

This can provide a convenient one-seat ride.

However, it must be clearly communicated.

A passenger should not have to guess whether the bus continues or whether everyone must get off.

Destination signs, announcements, timetables, and operator instructions all need to agree.

Delays Can Spread

Interlining has a major disadvantage.

Suppose Route A regularly encounters afternoon congestion.

If the same bus becomes Route B immediately afterward, Route B may begin late.

The passengers on Route B may have no idea that their delay began several miles away on a completely different route.

This is called delay propagation.

Recovery Becomes Important

Proper recovery between interlined trips can reduce that problem.

If the block provides enough time between assignments, a slightly late Route A trip may still allow Route B to begin on time.

If the connection is too tight, the two routes become operationally dependent on one another.

Not Every Pair of Routes Is a Good Match

Successful interlining generally requires compatible:

• Locations
• Running times
• Vehicle requirements
• Passenger demand
• Terminals
• Schedules

Two routes may look convenient to combine but create operational problems when examined in detail.

Destination Signs Matter

Interlining also highlights the importance of destination signs.

At the end of one trip, the display may need to change to:

• A different route number
• A different destination
• A special or short-turn reading

If the wrong sign is displayed, passengers may board the wrong service or avoid a bus they could have used.