Colelagues
This is to let you know of the next CTS seminar at UCL
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Seminar
11 February 2009
16.00, University College London
B05 Chadwick Building
Centre for Transport Studies
Traffic flow on managed motorways
Puff Addison and Benjamin Heydecker
Abstract
Reduced speed limits are used on the M25 motorway at busy times as
traffic management measures.
The idea underlying this is that by reducing traffic speed, flow can
be improved and travel times reduced.
This raises the question of how slowing traffic down can result in
quicker travel. The study of traffic on
the managed section of the M25 has led us to a view of the
fundamental diagram that relates traffic flow
to its density that differs from the conventional one. First and most
important, we depart from the usual
requirement that the diagram has an intrinsic jam density that forces
speed and hence flow to zero.
Instead we view jam density as the density that occurs when speed is
constrained to be zero. We thus
reverse the usual causality in congested conditions. This view is
supported, and indeed was motivated,
by investigating speed-density models with and without a fitted jam
density. Extending this observation
we suggest that in congested conditions speed determines density
rather than vice versa. Motivated by
this we examine the fitting of models where speed is the explanatory
variable and occupancy the
dependent variable; we show that this formulation is useful in
evaluating variable speed limits when they
are used as a traffic management measure to improve the flow of
traffic. A second outcome of our
preferred speed occupancy model is the existence of a convex region
in the fundamental diagram within
the congested part. This convex region allows the existence of stable
start waves, which are observed
in practice.
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Benjamin Heydecker
Professor of Transport Studies
Centre for Transport Studies
University College London
Gower Street
LONDON WC1E 6BT
England
Phone: 020-7679 1553
Fax: 020-7679 1567
e-mail: [log in to unmask]
www.cts.ucl.ac.uk
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