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Chapter 2 | Prevailing strategies to manage congestion

Box 2.2

San Diego’s Interstate 15 Express Lanes

In October 1988, three reversible HOV lanes, for exclusive use during peak

periods by car-pools (2+), van-pools, buses and motorcycles, were opened

down the median strip of Interstate 15, one of the fastest growing freeways

in San Diego. a In 1998, the lanes became available for driver-only vehicles, in

return for a toll payment. This was one of the first HOT lanes in the US. Tolls

vary with the level of congestion on the HOV lanes, with changes possible

every six minutes to maintain traffic-flow targets on HOT lanes. Variable

message signs located in advance of the entry points post the current charges,

which generally vary between US$0.50 and US$4 but can be as high as US$8 if

demand is high.

On average, approximately 75 per cent of the weekday traffic using the priced

HOV lanes goes for free (vehicles with two or more occupants qualify as

HOV). b

a. While California’s 91 Express Lanes are often cited as the first example of a HOT lane in the US,

it does not provide an example of a conversion from a HOV lane to a HOT lane, as it was built

as the latter.

b. For more details on San Diego’s Interstate 15 Express Lanes see Business, Transportation and

Housing Agency, California (2002).

whether this outcome is an improvement over all general-purpose lanes. The answer

will be location-specific.

In conclusion, in the researched applications of the system to date, network

congestion increases because the HOV lanes are under-utilised. Studies also show

that much of the observed usage of HOV lanes is by groupings of existing HOVs onto

that lane rather than an actual behavioural change (which is needed if congestion is

to be ameliorated).

Increasing public transport usage

The other major policy strategy aimed at reducing demand for road capacity

involves promoting public transport as an alternative mode. Improvements in public

transport are often coupled with congestion charging. Subsidising public transport

is commonly justified on the grounds that it reduces congestion.

The logic of promoting public transport usage is straightforward: if some motorists

shift to public transport there will be fewer cars on the road and hence less congestion.

Public transport use in preference to private motor vehicles is also advocated as

having added health benefits: reduced emissions as congestion declines; and because

public transport users tend to walk more than car users.

However, in many cities, public transport has declined to become only a small

proportion of trips. Furthermore, it has proved difficult to achieve a significant

mode shift from car to public transport as public transport usually provides a poor

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