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Volume 4 • Issue 5 • 1000160
J Civil Environ Eng
ISSN: 2165-784X JCEE, an open access journal
Civil & Environmental Engineering
Keymanesh and Haghighatpour
J Civil Environ Eng
2014, 4:5
Review Article
Open Access
Calculation of Sight Triangle Dimensions and Unobstructed Area at
Railway Level Crossings in Iran
Mahmoud Reza Keymanesh
and Pegah Jafari Haghighatpour
Faculty of Civil Engineering Division, Department of Engineering, Payame Noor University (PNU), Tehran, Iran
Ph.D Candidate in Transportation of Tehran Payame Noor University, Tehran, Iran
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 a
t railway track, road or any speeds change in calculations.
*Corresponding author:
Pegah Jafari Haghighatpour, Ph.D Candidate in Trans
portation of Tehran Payame Noor University, Tehran, Iran, Tel: 09125996931; E-
October 24
, 2014;
November 07
, 2014;
, 2014
Keymanesh MR,
Calculation of Sight Triangle
Dimensions and Unobstructed Area at Railway Level Crossings in Iran.
J Civil
Environ Eng
4: 160. doi:
, 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.
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:
Figure 1:
Crossings classification in Iran railway.
Volume 4 • Issue 5 • 1000160
J Civil Environ Eng
ISSN: 2165-784X JCEE, an open access journal
Keymanesh MR,
Calculation of Sight Triangle Dimensions and Unobstructed Area at Railway Level Crossings in
J Civil Environ Eng
4: 160. doi:
Page 2 of 5
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 grade
, service level
of way
, and driver expectation
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:
It is determined according to AADT and the vehicle speed
Indeed, upgrading the level crossing over the road service would be waste the
Sudden encountering of driver with level crossing will certainly lead to crash
Figure 2:
Various situations for approaching vehicle to crossing.