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Calculation of Sight Triangle Dimensions and Unobstructed Area at Railway Level Crossings in Iran | OMICS International
ISSN: 2165-784X
Journal of Civil & Environmental Engineering
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Calculation of Sight Triangle Dimensions and Unobstructed Area at Railway Level Crossings in Iran

Reza Moayedfar1, Mehdi Sharifi Moghadam2, Pegah Jafari Haghighatpour3*
1Faculty of Civil Engineering Division, Arak university, Iran
2Master of science in Civil-Transportation Engineering, Tehran, Iran
3Ph.D Candidate in Transportation of Tehran Payame Noor University, Tehran, Iran
Corresponding Author : Pegah Jafari Haghighatpour
Ph.D Candidate in Transportation of Tehran Payame Noor University
Tehran, Iran
Tel: 09125996931
Email: [email protected]
Received October 24, 2014; Accepted November 07, 2014; Published November 12, 2014
Citation: Moayedfar R, Moghadam MS, Haghighatpour PJ (2014) Calculation of Sight Triangle Dimensions and Unobstructed Area at Railway Level Crossings in Iran. J Civil Environ Eng 4:160. doi: 10.4172/2165-784X.1000160
Copyright: © 2014 Moayedfar R, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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The passage way of road vehicles, humans and domestic animals over rails is called “crossing” at railways. The most significant of crossing are roadway crossings and level crossings have particular importance among them. Railway level crossings safety influences on rail and road transportation safety. 10% of rail accidents occur on crossings and 10% of crossing accidents cause to death. This issue indicates the importance of crossing safety. Several factors is effective in crossing safety. For example, rail and road vehicles should be aware of approaching the crossing at a proper distance and have appropriate visibility to each other for safe passage through crossings. So line and road sight obstacles should be eliminated at specified distance. In this paper, we try to determine proper dimensions of unobstructed area for maximum speed 120 km/hr of passenger trains and maximum road speed limit 120 km/hr of Iran railway lines by examining previous studies in other countries, Iran railway regulations and international standards. Railway track and road should not have horizontal or vertical arc at this distance and to provide enough visibility, any obstacles including trees, buildings, fixed or mobile equipments and etc. should not exist in distances forming triangle. Also the effects should be considered if there is any slope in both sides of crossing at railway track, road or any speeds change in calculations.

Level crossings; Railway; Sight distance; Sight triangle; Unobstructed area
Increasing speed, comfort, environmental compatibility and economical saving are the goals which could be outlined in transportation engineering [1]. But also the most important goal of transportation engineering is providing safety and should give special consideration to it. Nowadays safety becomes as the most important influencing factor on transportation quality to legislators and transportation infrastructure users of our country. Also other effective factors on quality and transport properties are affected by severe influence of safety and if there is no safety, all other goals will also experience challenges. Transportation safety is affected by several factors which vehicles, user, environmental conditions and transportation infrastructures properties (including roads, railway tracks, airports, and etc.) are the most important factors among others. Transportation infrastructures designer’s considerably improve the transportation quality by providing safer designs.
Road and railway level crossings are intersecting of two different types of transportation and lack of safety of them will directly effect on safety of both types of rail and road transportation. Road and railway level crossings are the most black spots of both types of transportation (especially in rail transportation). Accidents which have occurred at level crossings were very tragic and terrible, while almost one of every 150 road accidents caused to death and one of every 10 vehicle - train crashes caused to death.
Additionally, the existence of these intersections leads to increased travel duration in both rail and road transportation sections. This increase will be changed according to crossing design and if there is no correct design, these intersections could create traffic nodes in roads.
Status of Road and Railway Interchange
The passage way of road vehicles, humans and domestic animals over rails is called “crossing” at Iran railway. The crossings are divided as legal and illegal groups in terms of legal considerations and they divided into level and unleveled as crossing method from two types of traffic.
Level crossing is a crossing in which road or pedestrian traffic intersects with level of passing rail traffic at same point. Also level crossings are divided into protected and unprotected groups. Road and rail intersections classified according to public regulation of Iran railways can observed in Figure 1.
The number of 243,016 level crossings that 63,387 crossing (equivalent to 42% of all crossings) equipped with automatic control equipment have been existed in United States in 2004.
The number of train collisions with road vehicles has increased 15% and the number of casualties 17% with an increase of 712 to 744 million trains per mile (equivalent of 4% growth) and road traffic increasing from 2.7 to 2.9 trillion vehicles per mile (equivalent of 7% growth) at crossings between 1999 and 2003 [2]. This indicated that the accidents increase exponentially with increase of rail and road traffic at crossings. According to statistics, about 10% of total accidents are crossing crashes in Iran [3]. Although the use of traffic control equipment and geometrical design optimization of crossings not completely prevent the occurrence of accidents, but can contribute to reduce them considerably. The main provisions to reduce accidents at crossings are as follows:
• Wall and fence the railway privacy and remove crossing
• Interchanging the crossing
• Change the road or rail path
• Construct new crossing according to general requirement
• Install special signs (active and inactive) for level crossings and other road traffic control equipments
• Provide training to road and rail users
• Choosing the toll man in place, monitoring, enforcement and prevention policies
Although the safe way to prevent entry of persons and vehicles into railway privacy is interchanging the crossing, but usually in Iran problems such as limited financial resources during the construction of rail projects, road traffic cross the railway, lack of interchange crossings economic justification, lack of national design regulations for level crossings and some designers are not familiar with rules cause to not provide any scheme for road cross the railway. Therefore, not only crossings will not be interchanged but also they don’t have enough safety as level crossing and become a threat to road and rail users. In addition, level crossings that created after line construction increase these threats.
Effective Factors on Road and Rail Intersections Design
Several factors influence in the choice of railway crossings. They include political (such as existence of specific facilities or buildings around the crossing), legal and social (such as priority of constructing road or railway, rural or urban population who access to intersected railway, legal and lawful implications, public expectations, respect of law among drivers), economical (such as feasibility or providing funds of interchange crossings and installing the require equipments on level crossings), engineering factors, and etc. Effective engineering criteria on choosing proper crossing type are divided into three categories:
• Basic requirements of the system: for Instance, based on the road network requires, there should be used unlevel crossings and if not possible level crossings with active signs on highways and main roads should be used.
• Basic requirements of locomotive and vehicle drivers for making decision: the level crossings have not enough safety if cases such as stop sight distance, minimum unobstructed stop sight distance, and sight triangle distance, and etc. not possible to satisfy these requirements. So they should be converted to unlevel crossings.
• Performance level of way is including: road grade1, service level of way2, and driver expectation3.
Driver and locomotive driver sight implications
The geometric characteristics of road and railway requirements to provide adequate visibility for driver and locomotive driver to their path and make enough time for safe behavior should be regarded. If the possibility of these requirements will not be provided, it should be compensated with installing equipment, change the crossing, railway or road route to other type.
Generally, situations where driver can faced with them are divided into three categories
Approach area: driver should recognize the existence of crossing in front of them. search and observe train or signs and make the most appropriate decision’s.
Irreversible area: it’s the area which drivers decide to stop according to train distance and speed at beginning. If there is not correct and on time decision to stop, tragic accident will be happen.
Hazard zone: the area which stopped or moving vehicle may have collision with stopped or moving train (Figure 2).
Three sight distances are considered in design:
• Sight distance of observing and stopping at crossing
• Sight triangle (the sight of approaching vehicles to crossing with each other): the driver should see the train when entering into the crossing and make appropriate decision to stop or pass and stop 4.5 m before approaching the crossing (from closest edge of rail).
• Sight distance of discharging the crossing: the driver should see the train as approaching to crossing that could be able to determine safe exit time from crossing and also exits the crossing on time.
Geometrical design of road near the crossing should have specific form which not attracted the driver attention so much and also provides sufficient vision and enough safety for driver and locomotive driver to pass vehicles through the crossing. In this paper evaluate dimensions of sight triangle and design of unobstructed area will be done (Figure 3).
Sight triangle consists of three distances below:
• Safe rail distance from crossing (dT): when there is train at crossing, the vehicle where is at beginning of irreversible area with design speed would pass irreversible and hazard zone and all the vehicle completely exit from that area (pass the exit stop line).
• Road distance of vehicle from crossing (dH) is the beginning distance of irreversible area to entry stop line.
• Distance between driver and locomotive driver eyes that located at dT and dH.
Constant values and abbreviations
The constant values and abbreviations which were used in the equations are as follows:
dT: sight distance of train to discharge the crossing, distance of train from crossing as observing the vehicle until the vehicle has enough time to discharge the crossing and pass from exit stop line before the train approach to crossing,
L: length of road vehicle (considered equal to 2 m),
D: Stop line distance to side rail (considered equal to 4.5 m),
VT: Train speed,
VH: Vehicle speed,
VG: Maximum speed of vehicle (considered equal to 2.7 m/s),
G: Absolute value of slope based on the percentage,
F: Road friction coefficient along the movement on wet pavement,
a1: Acceleration of vehicle speed (considered equal to 0.45 m/s2),
J: Sum of observation and reaction time include clutch release (considered equal to 2 second),
da: Distance which vehicle has maximum speed (considered equal to da = V2 G/2a1 = 2.72/ (2×0.45) = 8.1 m),
de: Distance between driver eyes and front bumper of vehicle (considered equal to 2.4 m).
Stop sight distances in sight triangle
Minimum distance of obstacle from track axle by according to kinematics gabari of train that inserted in UIC Standards is considered equal to 2245 mm (1645 + 600 = 2245) [4]. Thus, American Federal Highway Administration of Transportation ministry suggested that reasonable stop line distance should be equal to 4.5 m from external edge of rail by considering require space for installing barrier, intersection equipments and signs [5]. Also this value is illustrated with D in Figure 4.
In addition to, the driver has a good visibility of rail track, traffic signal and crossing to stop vehicle in a safe place away from the possible collision if there isn’t any visibility obstacle at this distance. Figure 5 indicated the train as approaching to crossing from the view of stopped vehicle driver at stop line.
Vehicle stop sight distance is sum of driver eyes distance from front bumper of vehicle (de), recognition and reaction distance, break and stop distance. Thus, this distance is calculated from equation 1 according to U.S.A DOT recommendation and 2-1-5 clause of road geometric design (161 publication) [7].
dH = 0.278Vv + (V2v/(254(F∓G))) + D + de(1)
Assuming that leading path to crossing has no slope and with including constant values in the above equation we have:
dH = 0.695Vv + 0.011V2v + 6.9(2)
For maximum limited speed VH = 120 km/hr, we have:
dH = 0.695 × 120 + 0.0111202 + 6.9 = 248.7m
Horizontal and vertical arcs of road should be considered out of this distance and longitudinal slope must be 0%, and also this distance could be included for installing the sign of approaching to crossing [8]. Therefore, the mentioned distance in circular No. 12062, 2006.03.18, by CEO4 of Iran railway will be appropriate, although the accurate value should be calculated based on road design speed [3].
Stop sight distance of train is the distance when locomotive driver observes the vehicle at beginning of irreversible area, the distance between train and the crossing is equal to it.So the vehicle has enough time to pass the crossing and its back bumper completely exits from the hazard area [9]. This distance is equal to vehicle travel time from point B to F multiplied by train speed, thus we have:
Assuming that leading path to crossing has no slope and with including constant values in the above equation we have:
Assuming that leading path to crossing has no slope and with including constant values in the above equation we have:
For maximum limited speed of vehicle (VH = 120 km/hr) and train (VT = 120 km/hr), we have:
dT = 272.3 m
According to train breaking distance for speed 120 km/hr is considered to 700 m [10] if the vehicle couldn’t pass the rail track and stop on it, train collision will be inevitable. Thus, above mentioned distance is used only for standard crossings. If there is any possibility of stopping vehicle on rail track, following equation is suggested to calculate the train sight distance:
dT = 700 + 0.28VTt = 700 + 0.7 × Vt(5)
For VT = 120 km/hr, above mentioned distance will be equal to 784 m [10,11]. It is necessary to remind locomotive driver that if observes the vehicle at point B to C after reaching the 784 m from crossing, reduces his speed to prevent crash when the vehicle stop on rail track. So, passing time from point B to C of vehicle is [12]:
During this time, train would pass 268 m (d = 0.28VTt = 0.28×120×8.03 = 267 m), so decision and stop distance will be equal to 516 m. by considering 2.5 second for decision and reaction of locomotive driver, breaking distance of train will be:
d” = 516-0.28VTt = 516-0.28 × 120 × 2.5= 432 m
According to issue Number 544-1O. Of UIC Standards [4], this distance is equal to breaking distance at 95 km/hr speed. Also, if we assume that breaking acceleration is -0.85, we have:
Therefore, it is enough that locomotive driver reduce their speed below 95 km/hr after entering to 784 m distance from crossing and increase the speed again if the vehicle would not stop and train passed the crossing.
Maintaining the sight triangle requirements and unobstructed area at railway level crossings is important to maintain clear visibility around level crossings to reduce the risk of collisions. In some countries like the U.S.A. and New Zealand, the prevailing sight distance guidelines are well justified and documented, whereas the reasoning of the Finnish guidelines is less clear. In all countries the required sight distance from the road to the track depends on the maximum speed of trains. In Finland and Sweden the required sight distance at level crossings of a single railway track depends only on train speed, and if the road crosses several railway tracks, also on the distance between the outermost tracks. We can improve and use these rules that help to clear visibility around level crossings to reduce the risk of collisions in Iran too. So it is suggested that it is enough to have only two sights distance requirements for level crossings -the sight distance from the road to the railway for road users who have stopped in front of the level crossing, and the distance at which road users approaching the level crossing must be able to see it. The third option would be the so-called approach sight distance, which defines the length of railway road users must be able to see from the stopping sight distance and which allows them to traverse the level crossing without reducing their speed if there is no train in sight. The suggested new sight distance requirements concern mainly passive level crossings, where there are no devices that automatically warn road users if a train is approaching. At active level crossings with barriers or sound and a light warning device, the sight distance from the road to the track should be as good as can be achieved without unreasonable effort, but not necessarily as long as for passive crossings. Furthermore, it is suggested that the new sight distance guidelines be applied first to new level crossings.
The possibility of visual information from the environment for driver should be obtained in order to provide secure movement. So sufficient vision towards the crossing and mobile objects should be provided for vehicle drivers. Sight obstacles should be removed from the limits of sight triangle around the crossing to provide enough vision for driver and locomotive driver towards each other and also to avoid facing with unexpected situations. It should be constructed out of the sight triangle if there is a need to construct facilities (like Toll room).
The measurement of this triangle road side is calculated by Equation (2). The value equal to 250 m if this value could not calculate for some points. The measurement of side of rail track is also calculated by Equation (4) according to recommendation from American Federal Highway Administration of transportation ministry. It is suggested to remind locomotive driver that reduces their speed below 95 km/hr (if vehicle speed is 120 km/hr) after entering to 800 m distance from crossing as observe the vehicle at 250 m distance from crossing (if there isn’t any barrier) and also increases their speed again after exiting the crossing but consider to Iran road and railway conditions.
1 It is determined according to AADT and the vehicle speed
2 Indeed, upgrading the level crossing over the road service would be waste the investment
3 Sudden encountering of driver with level crossing will certainly lead to crash
4 Chief Executive Officer

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