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1 Intersection Safety Hossein Naraghi CE 590 Special Topics Safety March 2003 Time Spent: 13 hrs.

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Presentation on theme: "1 Intersection Safety Hossein Naraghi CE 590 Special Topics Safety March 2003 Time Spent: 13 hrs."— Presentation transcript:

1 1 Intersection Safety Hossein Naraghi CE 590 Special Topics Safety March 2003 Time Spent: 13 hrs

2 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

3 3 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

4 4 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)

5 5 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

6 6 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

7 7 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

8 8 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

9 9 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 mph or higher, otherwise a stop sign is used

10 10 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

11 11 Rural Intersections (continued) Staggered t-intersections Are very effective in reducing both crash frequency and crash severity Sweden reported that paired t-intersections are 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

12 12 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)

13 13 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)

14 14 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

15 15 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

16 16 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

17 17 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

18 18 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

19 19 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:

20 20 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

21 21 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

22 22 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

23 23 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

24 24 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

25 25 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

26 26 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:

27 27 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

28 28 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

29 29 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

30 30 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

31 31 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

32 32 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

33 33 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

34 34 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

35 35 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

36 36 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

37 37 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

38 38 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

39 39 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

40 40 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

41 41 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

42 42 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

43 43 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

44 44 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

45 45 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 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 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

47 47 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

48 48 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

49 49 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

50 50 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

51 51 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

52 52 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

53 53 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

54 54 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

55 55 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


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