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Intersection Safety

Hossein Naraghi

CE 590 Special Topics

Safety

March 2003

Time Spent: 13 hrs

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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%

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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