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Traffic engineering began in America with a country's first traffic engineer in 1920's. Then, in the 1930's, the first institution for civil engineers was established. After that, traffic engineering as a science started to become very common in more and more countries. Britain is an example of one of these countries which also has an institution of civil engineers that defined traffic engineering in 1959 as: 'the part of engineering which deals with traffic planning and design of roads, of frontage development and of parking facilities and with the control of traffic to provide safe, convenient and economic movement of vehicles and pedestrians Ê¼ (Wells 1970).
Expressed in more simple terms, traffic engineering is an approach to transportation engineering which focuses on planning and operating roads efficiently and safely by controlling movement flows for vehicles and pedestrians by taking into account the three major elements in road safety which are road users (drivers or pedestrians), vehicles and roads. The efficient operation of roads leads to better time management and increased productivity for people. For example, if the road contains only one lane, but actually needs more than one lane, traffic engineering principles assert that traffic jams will be present.
As a result, these jams create a lot of delays for people who are using these roads. Then, these delays affect workers by decreasing work time and worker productivity. In addition, the efficient operation also prevents or at least reduces the amount of accidents and increases road safety. This is supported by a study that was performed by the Road Safety Foundation which suggested a safe road design to reduce around 33% of total road deaths and serious injuries in Britain (Road Safety Foundation 2008).
Intersections are the crossing of any two roads which can benefit from good operation as a part of road. This area of crossing involves road implements such as channels, traffic lights and direction marks. Also, intersections can be classified primarily as a type of separation. Clearly, there are two kinds of separation: vertical and horizontal. Bridges and subways are usually used on the area of vertical separation which is called the grade-separated intersection. The classification of horizontal intersection is divided into many types depending on the number of legs, controlling type of the intersection or intersection shape. This paper will discuss the classification of intersections types in some detail. Also, it will provide a short brief about intersection level of service which is used as an indicator to measure road efficiency. Then, intersections level of service are affected by some factors which also will be discussed within next lines. Finally, the most effective way to control road intersections will be presented as a conclusion for this study.
Classification of intersections types
Road intersections can be classified into two main approaches, grade-separated intersections which are separated in vertical direction by bridges or subways and at grade-intersections, which cross in the same level. This type can be classified into many types by the function which is used as the standard in classification.
Grade-separated intersection is an intersection which divides different traffic movements in the vertical direction. The different directions of movement can pass the crossing area without interfering because of the existence for each direction on different levels. Figure 2.1 show us different types of grade-separated intersections.
Figure 2.1 : The different types of grade-separated intersections (Garber & Hoel 2009, p.266).
At grade-intersections are defined as the area which different directions of movement are converged in the same level. This type of intersection can be classified into three main categories by function which is used in the classification. Shape, number of legs and type of control are the three functions for classification.
According to Flaherty et al (1997) in this classification, intersections can be classified into seven main forms which are T, Y, Scissor, Cross, Staggered, Staggered and skewed and Multi-way intersections. These different types are shown in Figure 2.2.
Figure 2.2 : The different types of shape intersections classification (Flaherty et al, 1997).
Number of legs classification
In this classification, intersections can be classified into three-leg, four-leg and multi-leg intersections. Notably, in my opinion, this classification is close to the previous method, but is more comprehensive. Figure 2.3 shows us the different types of this classification.
Figure 2.3: The different types of intersections by number of legs classification.
Control type classification
Intersections can be classified as unsignalized intersections, signalized intersections and roundabouts. These three types have minor types according to some geometrical and technical specifications. In the next lines, these minor types, when this classification is mentioned as the most common classification, will be discussed with a general overview on each major type.
This is the most common type of intersection in the areas with low traffic volume, which is controlled by signs such as "Stop," "Give Way" or "Yield". There are four types of unsignalized intersections. The first one has two roads, one of them is major and the other is minor. The priority for the movement on this type which comes from the major road and the stop signs usually is in the minor road. Then, the second type has two main or minor roads and each of them has the same level of importance. In this case, the stop signs will be in all approaches and the priority of passing for the driver who comes first with taking into account both of drivers must stop before passing. The third type has one major road and the other minor or offramp road and the sign allowing the minor to pass. Finally, the fourth type gives the priority to the one who comes first from the right of the way. Figure 2.4 shows an example of signs that control unsignalized intersections (Papacostas & Prevedouris 1993, p.232-33).
Figure 2.4 : Unsignalized intersections control sign (FreeFoto 2003).
A signalized intersection is an intersection that is controlled by a traffic signal. The traffic signal is the device which distributes or separates the time for each direction of movement in the intersection by different light colors. There are three colors: Red, Amber and Green. Each color indicates a different order. Red implies "Stop" to the direction of movement that it appears to. Amber implies "Get ready to stop" or "To go" in some countries to the direction of movement that it appears to. Green implies "Go" to the direction of movement that it appears to. These are classified as fixed or pre-timed, demand-actuated and semi-actuated signals. Firstly, the pre-timed signal is a signal with a fixed shared time on each approach. The amount of this time is called cycle length, which usually includes the green, amber, red and all red time on one approach. All red time means the time which all signals in the intersection have a red light. These times are programmed for the intersection depending on some traffic functions like traffic volume and road capacity. Secondly, the demand-actuated signal is a signal that is operated by a detector that responds to pedestrians who want to pass the intersection or vehicles that want to cross the intersection. It is usually used for two major roads with a similar traffic volume and level of importance. Thirdly, the semi-actuated signal is a signal that is operated for the intersections which have a different traffic volume with one major road and another minor road which the detector is placed on. The major road usually has a permanent green light unless there are pedestrians who want to pass the intersection or vehicles in the minor road that want to cross the intersection (Papacostas & Prevedouris 1993, p.193). These are the three types of signals which operate the signalized intersections.
The roundabout intersection is an intersection that has a central circular island. The movement from all approaches in this type of intersections usually uses one circular way to cross or pass to the other direction. Roundabouts are usually used for low level traffic volume or for high level traffic that needs to pass to the right direction (left in Britain). There are three main types which are used in Britain: normal, mini- and double roundabouts. The normal roundabout is a roundabout with a circular island that has a diameter of four meters or more. The mini-roundabout is a roundabout with a circular island that has a diameter of less than four meters. A double roundabout is a roundabout with two mini islands (Flaherty et al, 1997). Each type of these has special characteristics and usage. Figure 2.5 shows a typical four-leg roundabout intersection.
Figure 2.5: A typical four-leg roundabout intersection(Beyond the frame 2010).
Finally, these classifications can come together and make one intersection. For example, there is three-leg signalized T intersection. Flaherty et al (1997) support this method and summarizes the classification for intersections:
Intersections can be divided into the basic forms shown in Figure 2.2. From a design aspect these intersections can be divided according to weather they are uncontrolled, priority controlled (Stop, Give way), space-sharing (i.e. roundabouts), time sharing (i.e. traffic-signal controlled), or grade-separated (including interchanges).
To evaluate the intersection efficiency, the measures for this should be studied and also the factors affecting the intersection performance and control should be provided. This chapter contains these measures and factors.
The measurement of intersections efficiency
The traffic movement types on intersections should be known when road intersections are studied. There are three types of traffic movement as shown in Figure 3.1, which are crossing, merging and diverging movements.
Figure 3.1: Traffic movement types on intersections.
These types of movement are important when the road intersection is under design because of the relationship between it and conflict points which exist when two directions of movement interfere with each other. The maximum possible number of conflict points that occur to a four-leg unsignalized intersection are 32 points, which are shown at Figure 3.2 (Garber & Hoel 2009, p.331). The number of conflict points is usually used to describe general intersection efficiency when designing an intersection.
Figure 3.2: The number of conflict points for four-leg unsignalized intersections (Garber & Hoel 2009, p.333).
However, there are exact measures to indicate intersection efficiency for each intersection type depending on the controlling type. The intersection level of service can be measured for unsignalized, signalized and roundabout intersections (LOS). Also, the signalized intersection efficiency can be measured by intersection capacity utilization (ICU). In addition, the grade-separated intersection efficiency can be solved by the highway level of service.
Unsignalized, signalized and roundabout intersection level of service (LOS)
This is the most common method that is used to measure intersection efficiency, which is a function on delay time for vehicles that cross the intersection and is measured by the unit of second per vehicle. There are six levels that describe intersection situations; level A describes the best situation and level F describes the worst situation. Each type of intersection has different parameters that formulate delay time. Also, there are two formulas for unsignalized intersections. One is for a two-way stop controlled intersection which has one major and other minor roads and another is for an all-way stop controlled intersection which has two roads with the same importance. The delay time of two-way stop controlled intersections is control delay for the vehicles in the minor road and left-turn approach in the major road. Then, the delay time for all-way stop controlled intersections is control delay for the intersection as a whole. According to the Highway Capacity Manual (HCM), Table 3.1 provides the level of service with delay time for unsignalized intersections (HCM, 2000).
Table 3.1: The LOS with delay time for unsignalized intersections (HCM, 2000).
Avg. delay time (s/veh)
10 - 15
16 - 25
26 - 35
36 - 50
Then, the delay time for signalized intersections is the difference between the actual and ideal duration for the vehicles which cross the intersection. Also, according to the Highway Capacity Manual (HCM), Table 3.2 provides the level of service with delay time for signalized intersection (HCM, 2000).
Table 3.2: The LOS with delay time for signalized intersections (HCM, 2000).
Avg. delay time (s/veh)
10 - 20
21 - 35
36 - 55
56 - 80
The delay time for roundabout intersections is control delay for the intersection as a whole and also almost the same for unsignalized intersections. According to the Highway Capacity Manual (HCM), Table 3.3 provides the level of service with delay time for roundabout intersections (HCM, 2000).
Table 3.3: The LOS with delay time for roundabout intersections (HCM, 2000).
Avg. delay time (s/veh)
10 - 15
16 - 25
26 - 35
36 - 50
Finally, the delay time for signalized intersections infers the high efficiency of it. For example, level of service F has a delay time in signalized intersections which equals more than 80 (s/veh) while it equals more than 50 (s/veh) in unsignalized and roundabout intersections.
Intersection Capacity Utilization (ICU)
Intersection Capacity Utilization is a method for measuring signalized intersection efficiency which is based on traffic volume and intersection capacity as the main elements that describe the intersection situation. There are eight levels on this method; level A describes the best situation and level H describes the worst situation. Table 3.4 provides ICUs for each equivalent level of service (Husch & Albeck 2003).
Table 3.4: The LOS with ICU for signalized intersections (Husch & Albeck, 2003).
55.1% - 64%
64.1% - 73%
73.1% - 82%
82.1% - 91%
Highway and freeway level of service
The level of service for highways and freeways can be applied for grade-separated intersections because of the similarities between them and the sequence of movement. The level of service of highway is the ratio between volume of traffic and capacity of road (v/c). It depends on operation speed for the road. According to the Highway Capacity Manual (HCM), Figure 3.1 shows us the relationship between (v/c) ratio and operation speed and highway level of service (Papacostas & Prevedouris 1993, p.169-171).
Figure 3.1: The relationship between (v/c) ratio and operation speed and highway level of service (Papacostas & Prevedouris 1993, p.170).
Some factors affecting intersection efficiency and control
To choose the most appropriate way to control road intersection and to make the chosen way with high performance, there are many factors affecting this selection that the designer should take into account. The next lines provide some of these factors because intersection efficiency and control are important in studying road intersections.
Traffic volume is defined as the number of vehicles that pass certain points in the road during a specified time. It is usually taken on the peak period which has the maximum amount of traffic. Then, unsignalized, signalized and roundabout intersections are affected by increasing the delay times when the traffic volume increases. Also, the speed for vehicles that pass the highway is decreased when the traffic volume increase which naturally affects the highway's level of service.
Road capacity is defined as the maximum traffic volume which is loaded by the road. When the road capacity increases, the delay times for unsignalized, signalized and roundabout intersections will decrease and the highway vehicles' will increase. As a result, the intersection level of service will increase.
Geometric design, which includes intersection dimensions, curves, number of lanes and other factors, affects road capacity, which is related to intersection level of service. Also, it limits the designer when he decides to change the control type from unsignalized to a roundabout intersection or from signalized to a grade-separated intersection.
Human factors are defined as those factors which are related to human beings (drivers and pedestrians) who use the intersection such as age, vision and behavior. It affects the level of service directly by speed of vehicles, for example, and indirectly by geometric design considerations.
Topographical factors are defined as the factors which are related to the earth forming such as highlands and lowlands. These factors also affect the level of service directly by speed of vehicles, for example, and indirectly by geometric design considerations. Also, topographical factors affect the designer's decisions when planning the intersection.
The cost of controlling the type of the intersections is an important element to consider. Then, all intersections can be controlled by grade-separating to ensure that traffic movement will be without interruption. The cost should be considered when the intersection designer decides to change from an unsignalized intersection to a roundabout or signalized intersection and from roundabouts to signalized intersections and from signalized to grade-separated intersections.
Selection and conclusion
After providing intersection classification ,according to the type of control that is the most important classification in this paper, which are unsignalized, roundabout, signalized and grade-separated intersections. Also, this paper discussed the measures that are used to evaluate intersection efficiency which are level of service (LOS) for unsignalized, roundabout, signalized and grade-separated intersections and intersection capacity utilization (ICU) which is used for signalized intersections. The intersection level of service (LOS) is a function of delay time for unsignalized, roundabout and signalized intersections and it is a function of speed for grade-separated intersections. Traffic volume is a main factor of affecting intersection efficiency for all of intersection types. It can be concluded that the selection of the most effective way to control road intersection depends basically on traffic volume and cost. It also depends on less important factors such as road capacity, geometric design and human and topographical factors. In my opinion, the controlling type on the intersection can be represented as a life cycle. In this cycle, the intersection started with unsignalized controlling and finished, if the traffic volume increased, by grade-separation. This is subject to the suitability of other factors such as geometric design. Figure 4.1 demonstrates a simple picture of the most effective way to control road intersections.
Figure 4.1: The most effective way to control road intersections.
For example, there is a roundabout intersection with a delay time of more than 80 s/veh (level of service F). That means the intersection has a high traffic volume which will cause jams and affect traffic movement. Therefore, this intersection should be changed to a signalized intersection to decrease this delay and service traffic with a higher efficiency. However, the question is whether controlling this intersection with pre-timed or demand-actuate signals is better. The answer is subject to other factors that are potential subjects for other research. Finally, this is a general principle which is used to select the most suitable way to control road intersection. It also depends on some detailed calculations to design each intersection for a specific place.