A bus bulb, also called a bus boarder, bus border, bumpout, bus cape, or a kerb outstand is an arrangement by which a sidewalk or pavement is extended outwards for a bus stop; typically the bus bulb replaces roadway that would otherwise be part of a parking lane. With bus bulbs or boarders, a bus can stay in its traffic lane to discharge and pick up passengers, instead of having to pull over to the curb.

The term bus bulb is prevalent in North American usage, whilst bus boarder or bus border is used elsewhere.

A bus bulb or boarder can be considered as a specific form of curb extension, although that term is more normally used to describe a sidewalk extension for the purposes of traffic calming or other traffic management purposes.

Benefits include preventing buses from being delayed by having to pull back into traffic, reducing risk of traffic collisions, reducing pedestrian exposure in crosswalks (if provided at the same location), reducing sidewalk congestion, providing space for bus patron amenities including bus shelters, and traffic calming. The protrusion also facilitates easier full length alignment of a bus entrance with a raised kerb stop, especially to allow level boarding in the case of low-floor buses.

Bus bulbs also retain more parking when compared to a bus stop located in a parking lane, as a bus stop so located requires run-in and run-out tapers. Together these tapers may take up as much space as the actual stop, requiring parking to be prohibited over a longer length of road than with a bulb, where cars can park immediately on either side of the bus stop itself.

Standard bus bulbs can create safety hazards for cyclists. In a standard bus bulb design, the narrowing of the road can create danger for cyclists by making them merge back into traffic.

To resolve this issue, multiple designs exist to integrate bike lanes into bus bulbs. The National Association of City Transportation Officials recommends two types of modified bus bulbs that integrate with bike lanes: "boarding islands" and "shared boarding areas." The "boarding island" design routes the bike lane behind the bus bulb, and the "shared boarding area" design uses a ramp to route the bike lane through the bus bulb. Both designs require cyclists to yield to passengers boarding and disembarking at the stop.

Where more than one lane exists in one direction, modelling has shown that there is no additional delay overall for following drivers, since the buses have shorter dwell times (there is less time spent pulling back into traffic) and since following vehicles may use the second lane for passing.