Article
citation information:
Alrawi, F. Measuring the relative
importance of applying engineering solutions to urban traffic intersections: a
planning perspective. Scientific Journal
of Silesian University of Technology. Series Transport. 2018, 100, 05-13. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2018.100.1.
Firas ALRAWI[1]
MEASURING THE
RELATIVE IMPORTANCE OF APPLYING ENGINEERING SOLUTIONS TO URBAN TRAFFIC
INTERSECTIONS: A PLANNING PERSPECTIVE
Summary. This research is an
attempt to compare engineering solutions for traffic intersections from a
planning viewpoint. In this research, various solutions have been discussed for
traffic intersections (traffic light, roundabout, underpass and overpass). The
research highlighted the importance of each of these solutions in urban
environments and clarified all variables related to the pros, cons, costs,
capacity and environmental compatibility of each of these solutions with the
surrounding urban environment. Weights were developed for all these variables,
then correlation was determined using the linear regression method.
The analysis of statistical results shows that the
creation of underpasses often achieves most of the designated goals when
compared to other solutions, despite some technical difficulties and high
construction costs.
Keywords: transportation
planning; traffic intersections; traffic light; roundabout; underpass;
overpass; linear regression.
1. INTRODUCTION
The urban transport network is
congested with vehicles, especially at intersections. There are many points of
conflict between cars at these intersections, and the number of conflict points
varies depending on the type of intersections and how they are resolved; see
Figure 1 [1].
Roads sometimes share the same area at
intersections. All drivers, when they reach intersections, must select an
alternative direction to proceed. Intersections may be classified as either
separated with ramps or slopes (interchanges) or non-separated.
When traffic volumes are high,
structural interchanges are usually implemented, which provide different levels
of traffic crossflow without interruption, to reduce delay and conflict points
at intersections [1].
The increase in population and
vehicles and the expansion of urbanized cities, such as the city of Baghdad,
have led to a greater impacts on road networks and intersections, which have
resulted in the creation of crowded lanes of vehicles at these intersections.
This research studies a general
problem concerning Baghdad City, which has recently constructed seven
overpasses and one underpass at the city’s intersections [2] in order to
solve traffic congestion. However, these intersections have not functioned
properly.
Fig. 1. Conflict points at four-approach non-signalized intersection [1]
2. TRAFFIC INTERSECTION SOLUTIONS
To reduce traffic delays and
crashes at intersections and to improve the capacity of roads and streets, many
types of traffic control systems are used.
A traffic light is one of the most
effective traffic controlling methods at intersections. This kind of solution
can be used to eliminate many conflicts at the intersections because the
traffic streams are assigned at different times in the intersection [1].
The traditional traffic light
solutions have many advantages for the city in terms of the low cost of
construction and maintenance, being easy and quick to construct, flexibility in
potential replacement by other solutions, and spatial suitability of different
areas. Adding some improvements to traffic lights enables traffic to flow
quickly, easily and safely and increase their importance [3-6].
To monitor the characteristic and
volume of traffic in order to give priority to congested tracks and public
transport, sensors are added to traffic signal systems, which adapt traffic
lights in order to dynamically adjust to various situations, such as time of
day and specific traffic conditions [3-5]. The use of these smart traffic
signals has increased the efficiency of these systems, especially in areas with
irregular traffic [3]. The green wave is one of the improvements made to the
traffic signal in order to achieve continuity in the traffic flow and reduce
the overall duration of stops at intersections [7,8].
When traffic lights are unable to
regulate movement at intersections, roundabouts may be constructed to help
solve a number of major and minor road problems encountered by traditional
intersections [9]. The decision-making process of creating a roundabout is a
planning more than an engineering process. Based on the existence of traffic
problems at several intersections in an urban area, it is necessary to study
volumes, patterns of traffic and adequate space availability [10],
as well as consider the central island radius and the level of the whole
intersection with access roads when using roundabouts [11].
Roundabouts decrease
vehicle-pedestrian and vehicle-vehicle collision points at intersections, which
helps reduce the number of accidents. In the US, studies found a dramatic drop
in accidents by 29% and injured persons by 81% when roundabouts were installed
at uncontrolled intersections [9,12,17].
In some instances, setting up
roundabouts is not feasible for several reasons, such as a lack of space
especially in historical and highly developed areas, when the land‘s
nature does not fit with roundabouts, or in areas where there is the overlap of
high traffic volumes with pedestrians and bicycles. Thus, transport planners
and engineers are resorting to alternative solutions, such as overpasses and
underpasses (interchanges) [13].
The overpass and underpass are
considered appropriate planning alternatives when the traffic volume at
intersections and roundabouts increases [14,20].
It is clear that the main reason
for the hesitancy to create an overpass or underpass is cost. But, in the
future, these structures will positively impact the city economy by providing
greater accessibility [15,18].
Underpasses are usually costlier
than overpasses when only considering the construction factors. But it may be
argued that the additional costs incurred during construction do not outweigh
the benefit that the underpass will provide by utilizing structures above it.
Reconstructing an intersection to grade-separate left turns requires an
extension of the intersection footprint. This means introducing an additional
right of way and adding some supplementary costs to the project, as well as
creating impacts, especially in terms of the city plan [15]; see Figure 2.
Fig. 2. Boston‘s Big Dig, which provided Boston
with “a new highway system that has made zipping beneath Boston and
Boston Harbor more easy” [16]
3. METHODS
For the purpose of comparing a
range of traffic solutions for urban intersections (traffic lights,
roundabouts, overpasses and underpasses) in an urban environment, it is
necessary to establish a set of variables (Xs) representing the strengths and
weaknesses that should be considered when addressing issues at intersections.
These variables are related to the
location of intersections in the urban environment, the type of land used, the
cost of land, construction cost and other factors. Based on the experience of a
group of transport experts (academics and technicians), weights have been put
in place to represent the relationship between the solutions for intersections
and the variables (Xs). These weights varied between 1 and 10, with 10
representing the highest positive value, while 1 represents the lowest negative
value between loads.
The relative importance
of achieving each of these variables (Xs) in the city was determined by the
same weights (1-10); see Table 1.
The application of the linear
regression method to this data demonstrated a correlation between the relative importance of the comparison variables (Xs), in relation to
the intersection solutions.
It was found that many factors
affect the selection of one of these solutions at an intersection of the city,
which can aid transport planners and decision-makers in determining the
fundamental features of the future intersection before developing plans. Additionally,
any intersection in the city can be tested for the purpose of determining the
appropriate solution for it, depending on the relative importance of its own
conditions.
Table 1
Weights
of study variables Xs and their importance Y
|
Preference variables |
Importance |
Traffic light |
Roundabout |
Underpass |
Overpass |
|
X1 Traffic regulation |
8 |
5 |
4 |
10 |
9 |
|
X2 Decrease waiting time |
8 |
2 |
5 |
10 |
10 |
|
X3 Reduce fuel consumption and pollution |
8 |
2 |
4 |
10 |
9 |
|
X4 Reduce police efforts |
6 |
7 |
5 |
9 |
9 |
|
X5 Minimize accidents |
10 |
5 |
4 |
9 |
7 |
|
X6 Reduce traffic downtime |
8 |
2 |
6 |
10 |
10 |
|
X7 Remove direct vehicle collisions |
7 |
5 |
2 |
9 |
9 |
|
X8 Sustain urban mobility |
7 |
5 |
6 |
7 |
6 |
|
X9 Preservation of urban spaces |
8 |
6 |
8 |
10 |
1 |
|
X10 Property costs |
6 |
7 |
4 |
10 |
1 |
|
X11 Land development |
7 |
5 |
3 |
10 |
1 |
|
X12 Average lifespan |
6 |
7 |
8 |
10 |
8 |
|
X13 Malfunctions and delays |
5 |
3 |
9 |
7 |
10 |
|
X14 Expansion requirements |
8 |
7 |
5 |
10 |
1 |
|
X15 Suitable for high traffic |
8 |
1 |
2 |
10 |
10 |
|
X16 Construction between traffic lights |
5 |
8 |
2 |
6 |
6 |
|
X17 Duration of construction |
2 |
10 |
9 |
1 |
2 |
|
X18 Design and implementation difficulties |
1 |
9 |
9 |
2 |
4 |
|
X19 Construction and operating costs |
4 |
9 |
10 |
1 |
3 |
|
X20 Maintenance cost |
4 |
8 |
8 |
1 |
2 |
|
X21 Ability to discharge vehicles |
8 |
3 |
5 |
10 |
10 |
|
X22 Environmental impacts |
8 |
6 |
5 |
10 |
1 |
|
X23 Impact on city scenery |
8 |
6 |
6 |
10 |
3 |
|
X24 Interference with pedestrians |
7 |
5 |
1 |
10 |
7 |
|
X25 Suitable for historical areas |
8 |
10 |
4 |
10 |
1 |
|
X26 Suitable for irregular traffic flow |
8 |
9 |
2 |
10 |
10 |
|
X27 Suitable with slopes |
6 |
5 |
1 |
9 |
8 |
|
X28 Suitable for high cycling users |
7 |
6 |
1 |
10 |
6 |
|
X29 Resilience |
6 |
8 |
9 |
1 |
1 |
4. RESULTS AND DISCUSSION
4.1. Hypothesis
The application of the regression
method is based on the data in Table 1. In the case of estimating the selection
of H₀, there is no significant
regression and the dependent variable is not correlated with the independent
variables. In the case of H₁, there is a significant
regression and correlation between the dependent variable and independent
variables.
4.2. Discussion of the
model
The regression test results showed
strong correlation between the underpass solution and the relative importance
of the preference among different types of solutions.
The correlation ratio was (0.8),
which is a strong correlation, with a significant percentage of (0.000), which
is statistically significant. The underpasses represented more suitable
solutions for most urban areas, which are not visible, not consuming the
city‘s land, and are providing a high capacity for traffic volumes. There
is no correlation between the overpasses and the relative importance of selecting
the types of solutions that recorded a weak correlation (0.253), at (0.093),
which is non-significant value. The overpasses have a negative impact on the
urban environment and the city‘s scenery. In addition to the high
construction cost, limited urban resilience and incompatibility with historical
areas, overpasses also consume large areas of the city‘s land.
The type of the proposed solution
(traffic lights and roundabouts) was inversely correlated with the preference
variables. This indicates that these types of traditional solutions have become
less important and more burdensome to the city, because it represents conflict
points of traffic, congestion and low capacity for growing traffic volumes.
4.3. Best solution
Solutions with low correlation have
been excluded. The underpasses solution was proven in the data set to be the
best model due to its high correlation and significance. The
correlation-squared value (R²) was 0.64, while the adjusted R-squared
value was (0.63) and the value of the standard deviations, which were
interpreted as independent variables of 64% from the dependent variable, was
(1.2); see Table 2.
Table 2
Model summary (b): approved solution
|
Model |
R |
R-squared |
Adjusted R-squared |
Std. error |
|
1 |
.800(a) |
.641 |
.627 |
1.20372 |
a Predictors: (constant), underpass
b Dependent variable: importance
Table 3 shows the result of the
(ANOVA) test, where the F-value was 48.109 with a significant value of 0.000. Therefore,
we accept the alternative hypothesis, that is, there is a relationship between
dependent and independent variables; and, from the preference of coefficients
for the regression equation (see Table 4), it is clear
which factors are affecting the model, while the regression equation is as
follows:
(1)
Table 3
Relationship between dependent and independent variables: ANOVA (b)
|
Model |
Sum of squares |
df |
Mean square |
F |
Sig. |
|
1 Regression |
69.706 |
1 |
69.706 |
48.109 |
.000(a) |
|
Residual |
39.121 |
27 |
1.449 |
||
|
Total |
108.828 |
28 |
|
a Predictors: (constant), underpass
b Dependent variable: importance
Table 4
Coefficients for regression equation
|
Model |
Unstandardized
coefficients |
Standardized
coefficients |
t |
Sig. |
Correlations |
|||
|
B |
Std. error |
Beta |
Zero-order |
Partial |
Part |
|||
|
1 (constant) |
2.827 |
.591 |
|
4.785 |
.000 |
|
|
|
|
Underpass |
.474 |
.068 |
.800 |
6.936 |
.000 |
.800 |
.800 |
.800 |
Dependent
variable: importance
5. CONCLUSIONS
There is a set of characteristics
that makes the overpass alternative more preferable. These characteristics
relate to traffic safety, sustainability of the urban environment and
preservation of the city‘s land, as characterized by paucity, especially
in historical areas. Issues such as technical difficulties, duration of
construction and costs related to construction, management, and maintenance did
not have much impact on this alternative.
According to the study, the
relative importance of factors, such as urban resilience, was not so important,
due to the fact that the questionnaire was issued to people working in a
relatively stable environment against natural disasters. Factors such as
traffic congestion, inefficiency of public transport and high traffic accidents
increased the relative importance of variables related to those factors.
The availability of some financial
allocations related to the construction of road projects (costs factors) did
not make these factors difficult obstacles in the opinion of the identified
sample. Therefore, applying this model to different environments requires
different data relating to those areas.
It is not logical to transform all
the city intersections into underpasses, but this method should be seriously
considered. Instead of constructing several overpasses within urban areas such
as Baghdad, it is preferable to establish underpasses wherever possible to
reduce the urban deformity caused by concrete structures. As for traditional solutions to traffic intersections, traffic
lights and roundabouts are still practical and low-cost options in urban areas
with less traffic density. The conventional solutions have been inversely
correlated with the relative importance of the preference variables, showing a
strong individual relationship with some variables such as cost, duration of
construction and technical difficulties related to development. These are vital
but not at the expense of safety and the regulation of traffic in mega-cities
where it is difficult to solve intersections by conventional means.
Acknowledgements
To the Institution of International
Education, SRF, Saint Martin University’s: with
my love and appreciation.
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Received 28.05.2018; accepted in revised form 30.08.2018
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