Junctions

Road safety criteria in junctions design

The main objective of junction design is to increase convenience, comfort and safety while at the same time enhancing the efficient movement of all road users (motor vehicles, buses, trucks, bicycles, and pedestrians) [28].

Junctions are intended to operate where vehicles often must share space with other vehicles and pedestrians. Negotiating a junction requires many simultaneous or closely spaced decisions, such as selection of the proper lane; manoeuvring to get into the proper position; need to decelerate, stop, or accelerate; and need to select a safe gap. The following basic areas should be reviewed in conjunction with these decisions to produce a satisfactory design: junction angle; coordination of the vertical profiles of the intersecting roads; coordination of horizontal and vertical alignment for junctions on curves; improvement of operation, safety, and capacity through channelization; and drainage requirements for safe operation. Not only must the horizontal layout be carefully thought out, but the coordination of the vertical and horizontal alignment should be given more emphasis. Poor integration of these two elements often results in a junction that is less safe and uncomfortable to use [100].

An important safety aim is to match the speed at which drivers negotiate the junction with the complexity of the decisions to be made. This can be done, for example, by only allowing simple merging manoeuvres on high speed roads or by ensuring that drivers reduce speed on the junction approach (e.g. by deflection of path through a roundabout). Sight lines should provide drivers with sufficient information to make safe decisions, but not tempt them to try to select short gaps in conflicting traffic flows.

More specifically, the main design principles for junctions include:

• Minimization of traffic conflicts locations: A junction has a set of conflict points between vehicle paths, and a good design should aim at minimizing the severity of potential accidents at these points.

Figure 8 Number of conflict points at junctions and roundabouts

• Sufficient sight distances: Appropriate sight distances, both while approaching the junction area and being at the junction are of major importance for the safe operation of the junction. The creation of junctions in vertical sag curves is considered to be favourable. Another important parameter concerns the prompt perception and comprehension of the junction's layout and operation by drivers, particularly those who are not regular users of the junction, and therefore the selection of appropriate path and travel speed, on which drivers can be assisted by improved horizontal and vertical road marking and appropriate junction layout [49].
• Longitudinal section and transverse gradients design: It is important to design the longitudinal section of the road in the junction area and access areas, in order to achieve a smooth transition of transverse gradients and effective drainage. Ideal location for a junction is on a tangent. Location in curves may cause problems, as visibility is reduced, conflict points increase and superelevation and lane widening make the situation more complicated. Moreover, junctions should not have gradients over 3%, and never more than 6% in order to provide both improved comfort and sight distances; junctions should also preferably not be located at or near crest vertical curves [70]
• Left turning movements: The locations where left turns and U-turns are allowed should be specifically determined and appropriately configured, by means of traffic islands and/or marked lanes. It should be noted that left turns are high risk movements on a level junction. Research results in Great Britain have shown that these movements (right turns in Great Britain) are responsible for around 70% of all accidents on three-arm junctions [89]
• Minimization of weaving areas

Effect of junction type on road safety

The choice of a junction design depends upon several factors, whose relative importance varies between cases and should be assessed. The most important ones are [70]:

• Traffic safety
• Road type and function
• Number of concurring legs
• Traffic volume and type
• Design and operating speed
• Priority setting
• Terrain
• Available room
• Adjacent land use
• Service to neighbouring population
• Network considerations (design consistency)
• Environmental concerns
• Cost

The type of junction has to be suited to the road type, the environment and capacity, in order to maintain good readability both of the road and of the junction, as well as a satisfactory level of safety. According to the above, for example, junctions or roundabouts should not be used on motorways, and signalized junctions need not to be used on rural roads, except in very special cases. The following Figure 8 shows guidelines for the selection of junction type according to traffic flows.

Figure 9 Type of junction based on traffic flows (IHT 1997)

More specifically, the various types of junction present different advantages and limitations:

• Three- or four-arm non-signalized at grade junctions: These junctions may provide satisfactory road safety level when operating in low traffic volumes and speeds. Traffic islands and pavement marking, delimiting traffic directions and creating special lanes for left turning movements have a positive road safety effect [59]. When traffic volumes increase, it is necessary to establish traffic signals or consider modifications of the junction layout. In urban areas, changing a three- or four-arm level junction into a roundabout may lead to around 30% accidents reduction [91].
• Roundabouts. Roundabouts have higher capacity than three- or four-arm non-signalized junctions; Roundabouts appear to have considerable safety advantages over other types of at-grade junction and are now being widely used in many countries [63]. However, in some countries they appear to be related to higher accident involvement of motorized two-wheelers and bicycles [92].

• Signalized junctions. Signalized level junctions are the most common junction type in urban areas. Fatal accidents at signalized junctions are predominantly multivehicle [4]. The majority of accidents on signalized junctions concern left-turn vehicle movement or pedestrian's movement. Moreover, a higher accident involvement, in relation to their traffic volumes, may be observed for motorized two-wheelers and bicycles [92].

• Grade separated junctions (interchanges). These junctions present lower accident rates in general compared to at-grade junctions. It is noted that upgrading a three-arm non-signalized junction to a grade separated junction may result in a 50% accidents reduction, while the respective percentage for a four-arm junction may reach 75%.[89]. However, for the safe operation of a grade separated junction, an efficient planning is required. For example, research results have shown that reducing the number of lanes in the junction area, or designing weaving length of less than 1 km, may have negative safety impact. Moreover, accident rates on the arms of the grade separated junctions are about higher compared to the sections outside junction. Additionally, accident severity rates on junction arms are higher compared to the related rates for interurban roads (not characterized as motorways) [87].