The
measure of effectiveness of VMS with respect to congestion reduction and
mobility goal is the change (D OD vms ) in
overall delay between the scenario in which VMSs are operational in the corridor
(OD vms ) and
the scenario without the information provided by VMSs (OD b ): OD vms
= OD vms
– OD b. The
former scenario provides a baseline against which ITS elements are evaluated. In either scenario, the
overall delay is defined as the aggregate increase in travel time resulting
from the capacity reduction on the freeway. The following is the frame of our
evaluation model.
In
our model, Travel time of a motorist is modeled with the following four
components:
-
The traversal time on the portion of the freeway with reduced
capacity,
-
The merge delay: delay associated with the merging of traffic
on blocked lanes (if any) with traffic traveling on lanes that remain
unobstructed,
-
The queue delay: delay associated with the dissipation of
vehicle queues (if any) formed upstream of the incident location, and this
queue delay is based upon a deterministic macro traffic model.
-
The diversion time: travel time on an alternate route, for
motorists who made the decision to exit the highway corridor upstream of the
incident location and to divert to an alternate route.
The
network analyzed is a simplified freeway corridor presenting two route choices
to be considered by travelers: the first represents the route where the disruption
of free-flow traffic occurred while the second is a composite alternative
representing all the other routes motorists could divert to. See following
figure.
Suppose
an incident happens along the main corridor. Driver will choose to divert to
the alternative route or stay on the main corridor. The basic idea behind our
evaluation models is that the ITS components will supply traveler with
information with different details and therefore affect driver’s
decision-making. In our model, we just simply use one parameter, diversion
rate, to capture the estimated benefits, like delay saving, safety benefit,
emission saving and fuel consumption saving. Basically our model is sort of a
macro traffic simulation model.
My job is mainly to develop a package as an interface for
these evaluation models. And the developing environment is MicroSoft’s Visual
Basic:
Input & interface
General
parameters
General variables
Route type and Safety characteristics
Variables for
Value of time
Sample output
|
Delay benefit |
|
|
Inter-medium summary chart |
Safety benefits |
Safety
benefit increases with more travelers diverting to alternative route when the
diversion rate are small, but this does not make sense when the diversion rate
are large. This is one of the limitations of ITSOAM because the potential of
occurrence of accidents caused by congestion on the alternate route is not
fully considered in this model. |
Surprisingly, when the diversion rate is small
the environmental benefits decline with more travelers diverting to
alternative route. A possible explanation is diversion rate is too small to
reduce enough queue delay on the main to make up the increase of emission and
fuel consumption of traveling 15% longer. Only if the diversion rate is large
enough, environmental benefits tend to increase. This is consistent with the
trend on delay benefits. Environmental
benefits
|