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1
Intersection Safety
Hossein Naraghi
CE 590 Special Topics
Safety
March 2003
Time Spent: 13 hrs
2
Intersection Safety Intersections are the most critical element
of the road network At grade intersections are risky
Because different road users (vehicles, pedestrians, cyclists) are required to use the same space
Collision is only avoided if they are separated in time
In US over one-half of reported urban crashes and over one-third of reported rural crashes are at intersections
In Australia 43% of urban crashes and 11% of rural crashes are at intersections
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Intersection Safety (continued)
The main factors affecting safety at intersections Number of legs Angle of intersection Sight distance Alignment Auxiliary lanes Channelization friction Turning radii Lighting Lane and shoulder widths Right of way (rules, signs, signals) Approach speed Driveways
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Intersection Safety (continued)
In general as traffic flows and the ratio of minor to major road flow increases, more control is necessary for both safety and capacity reasons
A British guideline on appropriate intersection treatments classified by approach traffic volumes for both major and minor road flow is shown in Figure 9.1 (page 185)
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Intersection Safety (continued)
In increasing degree of standard and control, intersections are Uncontrolled
• Relying on a priority rule to indicate right of way Priority road
• Designated by Yield or Stop signs Roundabout Signal controlled
• Turning traffic filtering through on-coming traffic• Control of some or all turning movements
Grade separation
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Intersection Type Table 9.1 shows how crash rates varies with the
type of intersection and degree of control Things that influence the safety performance
Different configurations• Cross intersections• T-intersections
Different forms of control• Signals• Roundabouts
Different road functions• Major arterials• Minor arterials• Collectors• Local streets
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Intersection Type (continued)
From the data in Table 9.1 It can be seen following intersection
configurations tends to be safer than others• Roundabouts• T-intersections
Table 9.2 shows the appropriate intersection type in relation to the role of the intersecting roads in a road functional hierarchy
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Uncontrolled Intersections An uncontrolled intersection
The most basic form of intersection Relying on regulatory rule to resolve
priority between conflicting users Only applicable in very low volume
situations Requires establishment and maintenance
of a sight triangle• Enables vehicles on conflicting paths to see
each other
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Priority Controlled Intersections
Intersections of one major road with one or more minor roads Traffic on minor roads controlled with
stop or yield signs The decision to whether install stop or
yield sign is based primarily on sight distance consideration• In US a yield sign is used where sight
distances permit traffic on the controlled street to approach safely at 10-15 mph or higher, otherwise a stop sign is used
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Priority Controlled Intersections (continued)
Priority controlled intersections Are effective at low volume sites with low
approach speeds Should not over-utilized as this will likely
lead to disrespect Rural Intersections
Most intersections in rural areas are likely to be priority intersections
One particular treatment is the conversion of a cross intersection to a pair of staggered t-intersections
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Rural Intersections (continued)
Staggered t-intersections Are very effective in reducing both crash
frequency and crash severity• Sweden reported that paired t-intersections
are 1.5-2 times as safe as cross intersections for the same traffic flow
• US studies found that injury consequences is 1.5 times greater at cross intersections
It is preferable to orient the stagger such that the drivers cross the nearest traffic lane at nearly a right angle and then have unimpeded exit from the far lane
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For traffic driving on right, a left-right stagger is provided with a protected right turning area in the center of the major road
Figure 9.4a and 9.4b show the staggered t-intersections for the traffic driving on the left and on the right respectively
Rural Intersections (continued)
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Roundabouts A roundabout is a traffic control device
involving a one-way circulating roadway around a central island
Priority within roundabout is controlled by yield signs, although occasionally signal control may be used
Roundabouts are rare in US in compare to UK, principally because of the use of an onside rather than an offside priority rule (Todd, 1988, 1991)
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Roundabouts (continued) From an operational viewpoint, roundabouts
may be applicable: At intersections where traffic volumes lead to
unacceptable delays to traffic on minor road with stop, yield control or traffic signals
At intersections with high left turning volumes At intersections with more than four
approaches• Priority control may not resolve the situation• Signals may be less efficient due to the large
number of phases
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Roundabouts (continued) At intersections between collectors or
between a collector and a local street• Where disproportionally high number of crashes
occur On local streets
• To control speeds At rural cross intersections
• Where there is a crash problem involving vehicles on adjacent approaches or turning vehicles
At intersections where a main road passes through a rural town
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Roundabouts (continued) Where minor roads intersect at ‘Y’ or ‘t’
intersections• These involve high proportion of turning
vehicles
Roundabouts are less likely to be suitable where: A satisfactory geometric design can not
be provided • Restriction of space or topography
Traffic flows are unbalanced
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Roundabouts (continued)• High volumes on one or more approaches
which would dominate use of the roundabout Major road intersect a minor road
• Roundabout would cause unacceptable delay to the minor road traffic
Considerable pedestrian activities• High vehicle speed or heavy flows would
make it difficult for pedestrians to cross (unless pedestrian crossing facilities are provided)
At an isolated intersection in a network of linked signals
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Roundabouts (continued)• It is preferable to provide a signalized
intersection and incorporate it to the linked system to minimize delay, energy consumption and emissions
Peak period reversible lanes are used
Traffic flow leaving the intersection interrupted by a traffic control device• e.g. a pedestrian crossing could result in
traffic queues blocking the intersection
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Roundabouts (continued) Good safety record results from
Smaller number and spatial separation of conflict points
Control on approach speeds Low relative speeds at conflict points Simplicity of decision making for
drivers Good safety record can be
enhanced by:
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Roundabouts (continued) Provision of splitter islands on
approaches• Provides additional advance warning to driver• Give a good visual cue of the location of the
intersecting traffic flows Providing refuges for pedestrians
• Allow them to cross the road in stages
Safety problems can occur if: The merging angle is too sharp The roundabout is of unusual shape
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Roundabouts (continued) Signing is inadequate or confusing There are steep approach gradients There is adverse crossfall on the
circulating roadway There are slow-moving vehicles
• Bicyclists The deflection on approach is insufficient
to slow vehicles to a safe speeds• For safety, roundabouts with heavily flared
entries should have as much entry path deflection as possible
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Roundabouts (continued) Pedestrians and bicyclists safety at
roundabouts The major safety problem at roundabouts is
with bicyclists and to lesser extent with pedestrians• A British study found that 22% of crashes at
roundabout involved a bicyclist, compared with only 8% of crashes at signalized intersections
• Another study found that crash rates involving bicyclists at roundabouts in the UK were up to 15 times greater than that of cars at roundabouts
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Roundabouts (continued) The problem is one of a bicyclist circulating
within roundabout being struck by an entering vehicle (i.e. failing to give way)
Concern about the increased risk to cyclists needs to be seriously considered when weighing the benefits and disbenefits of adopting a roundabout treatment at a particular location
In some cases, bicyclists safety has improved following the replacement of a signal-controlled intersection by a roundabout, this was attributed to lower vehicle speeds
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Roundabouts (continued) Measures to improve bicyclist safety at
roundabouts Avoid squeeze points on the approach Ensure adequate deflection and speed control
• Speed should not exceed 30 mph Avoid large roundabouts
• Discourage high speed circulating traffic Avoid excessive width of the circulating roadway Ensure sight lines are not obstructed Consider provision of paths and ramps
• to allow bicyclists and pedestrians to bypass the roundabout by moving from island to island
Provide adequate lighting
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Roundabouts (continued) Pedestrians at roundabouts
Pedestrians are as safe at roundabouts as at other intersections by consideration of following factors • Provision of splitter islands
• Allow pedestrians to cross the road in stages
• Slower vehicle speeds The facility required for pedestrians
depend on the amount and intensity of their activities
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Roundabouts (continued) It may be necessary to provide a signal-
controlled pedestrian crossing adjacent to roundabout• It needs to be some distance away from the
roundabout to ensure that the traffic does not queue back and block the circulating roadway
• It may be necessary to provide a pedestrian fence to prevent pedestrians crossing the road away from the pedestrian crossing
Mutual visibility of pedestrians and motorists is important to maximize pedestrian safety, this can be enhance by:
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Roundabouts (continued)• Prohibiting parking on the approach to the
roundabout• Providing a higher level of street lighting• Ensuring signs and vegetation do not obscure
the view of pedestrians, particularly children Safety-effectiveness and cost-effectiveness
Except for situations involving significant numbers of bicyclists, a number of studies have shown that roundabouts are highly cost-effective in safety terms as replacement for stop or yield controlled intersections
• An Australian study found a 78% reduction in casualty crashes by installing roundabouts at low volume sites
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Roundabouts (continued)• Another study in UK found that the
installation of mini-roundabouts at existing priority controlled intersections can reduce crashes by 30-40%, and at existing signalized intersections can reduce fatal and serious crashes by 40-60%
The cost-effectiveness of an Australian roundabout installation indicates a benefit to cost ratio of 7.5 for crash saving alone, over the project life of ten years
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Traffic Signals By separating in time the use of road
space across major traffic flows, traffic signals have the potential to significantly reduce conflicts
Effects of signalization Under right circumstances, traffic signal
installation will reduce the number and severity of crashes• It is not clear whether road safety benefits will
result if the site has fewer than 3 casualty crashes per year prior to signalization
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Traffic Signals (continued) If signals are installed where the site
satisfies a safety warrant, some specific result can be quoted: A UK study based on 34 intersections found that
• Sites with more than 4.7 casualty crashes per year in the before period experiences a statistically significant reduction of 48%
• Sites with fewer than 4.7 casualty crashes experienced non-significant increase of 5.3%
A US study in the state of Michigan, based on 102 intersections, found a 15.5% reduction in total crashes
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Traffic Signals (continued) Controlled turns
Fully controlled left turn have a great safety benefits• Cameron and Foong 1991, examined 217
intersections approaches at which left turn phases had been installed
• Installation of fully controlled left turn led to a statistically significant reduction in all casualty crashes 45%
• Reduction of 82% in crashes involving vehicles turning from opposite direction
• 48% reduction in right angle crashes• 35% reduction in pedestrian crashes• 72% increase in rear end casualty crashes
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Traffic Signals (continued) The safety benefits of fully controlled turns
which have been indicated in several studies are due to clear and unambiguous direction which they present to drivers
A study examined the effects of the left turn phase on intersection capacity Full control of left turns yields poorer intersection
performance than partial control under virtually all conditions
The difference in performance is slight and unlikely to disprove the safety advantages
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Traffic Signals (continued) Advance warning
Advance warning are relevant where• A poor visibility of intersection
• over a crest vertical curve
• A high speed approach• The first signal after a long period of
uninterrupted flow conditions• Rural highway enters a city
A study in US concerning active advance warning devices for use at high speed approaches
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Traffic Signals (continued) Three devices were considered
• Flashing strobe light• Flashing RED SIGNAL AHEAD sign• PREPARE TO STOP WHEN FLASHING sign
Each of these were activated at a predetermined time in the signal cycle, usually at a certain time before the commencement of red phase
It has been concluded that the flashing RED SIGNAL AHEAD was the most effective device
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Enforcement The presence of surveillance reduces the
unsafe behavior of drivers Automated enforcement, using red light
cameras An Australian study of the effectiveness of
these cameras was undertaken• Crash data from 46 treated sites and 46 control sites
were analyzed• The result indicated that there was a 7% reduction in
total crashes and 32% reduction in right angle crashes at treated sites, this was significant at 5% level
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Signal design and operation
Major efforts being devoted to make existing signal safer
Following geometric and control characteristics have effects on safety Wider approaches and multiple lanes both
associated with higher crash rates for right angle crashes
Increasing the number of lanes at the holding line was associated with higher pedestrian crash rates
Longer approach sight distances were associated with lower crash rates for both left turning vehicles and pedestrians
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Signal design and operation (continued)
There are a number of aspects of signal design and operation which analyzed for their effect on safety Flashing green
• Aims to warn drivers of the impending end of the green period
• Introducing a flashing green phase in the last 2 or 3 seconds of the green period
• It has been used extensively in Israel• It cause significant increase in rear end crashes• There has been two possible responses to flashing
green, one being to stop and the other being to accelerate
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Signal design and operation (continued)
Starting yellow• A short yellow period before the start of the
green• A study about the effect of starting yellow on
driver response concluded that there may be a slight benefit in terms of reaction times and capacity
• No studies of its effects on safety have been found
Off-peak operation• In off-peak periods the practice in some places
is to have signal operate in a flashing yellow mode or flashing red mode
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Signal design and operation (continued)
• Done for mobility and energy reasons, not bringing a vehicle to a full stop
• There are safety disbenefits• A US study found that right angle crashes were
significantly over-represented at 4-legged arterial intersections when signals are in flashing mode during night time hours
• There was no significant change in rear end collisions
Mast-mounted signal heads• The practice is to use primary signals on the
upstream approach, secondary signals on the downstream side, in the median or on the far side of the road and tertiary on downstream but on the curb side
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Signal design and operation (continued)
• There are also overhead primary signals• To provide adequate advance warning of the
approach of the intersection itself• Give earlier indication of the signal aspect
• A US study 1991, confirmed the safety benefits of mast-mounted signal heads
• 63% reduction in right angle crashes• 25% reduction in total crashes
In some European countries, it is common to provide only the primary set of signals and have a small repeater set mounted on the signal post at driver’s eye level, since the first driver in queue cannot see the primary signals
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Signal design and operation (continued)
Demographic factors Different groups of drivers behave differently
in approaching to signalized intersections• Women are more likely to be involved in crashes
as a result of misjudgment or lapse of attention• Men are more likely to be involved in crashes as a
result of driving too fast• Higher risk taking is associated with
• Drivers with prior crashes or violations• Drivers with no passengers• Young drivers• Males• Drivers not wearing seat belts
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Signal design and operation (continued)
•Elderly drivers take significantly longer to respond, or to make the correct decision when confronted with complex signals such as multiple signal displays
•Elderly drivers also• have more difficulty in correctly handling
left turns• over-involved in right angle and rear end
crashes• Over-involve in crashes involving vehicles
turning from the opposite direction
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Channelization Channelization is the use of
Painted road markings Raised curbs Traffic islands Bollards
To guide vehicles along a specific path on the approach to and exit from an intersection Provides positive guidance to drivers Simplifies the movement
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Channelization (continued) Reduces the room for error Reduces confusion Separates the conflict points The number of decisions required for a driver at
any given instance is reduced Channelization installation or upgrading
have significant safety benefits An Australian study showed 26% reduction in
casualty crashes at signalized intersections and 54% reduction at non-signalized intersections
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Channelization (continued) The use of painted channelization at rural
intersections in Britain led to 35% crash reduction by protecting a turning vehicle and discouraging overtaking
US studies have found that the provision of exclusive left turn lanes at signalized intersections reduced crashes by 18-40 percent
County Surveyors’ Society in UK, reported that at rural intersections, ‘ghost islands’ (painted channelization had the potential to reduce crashes by up to 50%
46
Channelization (continued) Particular guidelines applicable to
channelization include Reduce the general area of conflict
• Causing opposing traffic streams to intersect at right angles
Merge traffic stream at small angles• Ensure low relative speeds between conflicting
streams Control the speed of traffic crossing or entering
an intersection • By alignment• By restricting width
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Channelization (continued) Provide a refuge for turning or crossing vehicles Avoid sudden and sharp reverse curves Reduce the number of islands to the minimum
necessary• Ensure safe and effective operation
Provide adequate curve radii and lane width for the prevailing type of vehicle
Provide explicitly for pedestrians and bicyclists Improve and clearly define alignment of major
movements Prohibit certain turns if necessary
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Grade separated interchanges
The key issues in safety aspects of interchanges are Configuration Traffic control Spacing
Heckman and Hayward 1992 found that Crashes on ramps and connecting roadways
increase with traffic volumes and with decreasing curve radius
Particular attention for the needs of trucks on ramps
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Grade separated interchanges (continued)
Uphill off-ramps have lower crash rates• Where possible it is preferable for the connecting
road to pass over the freeway Ramps that have lower safety performance
• Cloverleaf ramps• Scissor ramps• Ramps leaving from the median edge of the
roadway It is safer to diverge a given number of
entering vehicles at two or more on-ramps (or off-ramps) than at a single high volume ramp
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Railway crossing Crashes at railway crossing include
A train strikes a road vehicle A road vehicle runs into the side of a train Collision between vehicles on or near the
crossing• Associated with a vehicle taking action in
response to an approaching train or activation of a warning system
Crashes involving a vehicle colliding with the crossing furniture
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Railway crossing (continued) Safety devices
Elimination of crossing• Grade separation
• Overpass• Underpass
• Grade separation justifies road capacity and delay, rather than safety
Active control devices• Gates• Boom barriers
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Railway crossing (continued) Active warning devices
• Visual• Audible
Passive warning devices• A range of warning signs
• Crossbucks• Advance warning signs• Pavement markings
Visibility• Driver must be able to see the crossing and
its warning or control devices
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Railway crossing (continued) Pedestrian devices
• Pedestrian booms or gates activated along with those provided for vehicular traffic
• Additional warning and visibility for pedestrians crossing railway tracks
• Grade separated pedestrian facilities Geometric design of crossings
• The road and the railway should intersect at right angle or close to it
• Crossing should not be located on a horizontal curve
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Railway crossing (continued)• The vertical alignment at the crossing should
be as level as possible in the interests of• Sight distance• Rideability• Braking• Acceleration
• The road cross section should be continuous across the crossing
• Lane and shoulder width, and median provision should be maintained to avoid either a pinch point or the introduction of a roadside hazard
• Sight distance is a critical consideration at any crossing
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Railway crossing (continued) Following factors effect the selection of
safety device Type of highway Road traffic volume Number of trains Speed of trains Speed of road traffic Number of pedestrians Crash record at the site Sight distance Geometry of crossing Number of rail tracks Number of buses using the crossing Use of crossing by trucks carrying hazardous materials