Civil461 - CAS Report

UNIVERSITY OF AUCKLANDDEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERINGImprovement Options for Bell Ave/Mt Richmond Drive Cross-Intersection

Executive Summary

Table of Contents

1.0 Introduction1.1 Background of SiteThe site is a priority cross intersection between Mount Richmond Drive, Bell Avenue and Great South Road and is located in Mt. Wellington in the Auckland Isthmus. It was identified using the crash analysis tool CAS that this site required investigation due to a high number of crashes causing serious or minor injuries. A total of nine injury crashes were reported at the site from 2006 to 2010 causing four serious injuries and thirteen minor injuries. A further 19 crashes were also reported at the site in the same period which did not cause any injury. The typical accident rate AT for a similar site is 0.146 crashes per year. The site specific accident rate AS for this intersection is 1.48 crashes per year (refer Appendix _), which is 10 times the typical accident rate. The CAS database shows that 61% of the reported crashes were Crossing/Turning crashes, 21% were Rear End/Obstruction crashes, 11% were Overtaking Crashes and 7% were due to vehicles losing control or head on crashes. From the above data, it is clear that a significant proportion of the crashes are due to turning manoeuvres made at the intersection. The other crashes also appear to be caused directly or indirectly by the vehicles turning.

1.2 Description of SiteThe site is located in a Business Activity Zone as defined by the Auckland Council District Plan. Great South Road is a major arterial road which runs from Manukau to New Market. The section of Great South Road near the intersection has an AADT of 30729 vehicles/day. The two minor roads that join the intersection include Mount Richmond Drive and Bell Avenue and these both have an AADT of 162 vehicles/day and 179 vehciles/day respectively.

The intersection is situated on a generally flat area of land with the site gradient approximated to be -3% when travelling north on Great South Road and +3% travelling south on Great South Road. The approaches from Bell Avenue and Mt. Richmond is relatively flat. Great South Road is a four lane road with two lanes for northbound traffic and two lanes for southbound traffic. Both Mt. Richmond Drive and Bell Avenue are two lane roads with a single lane in each direction. The general road layout at the intersection and dimensions of the roads are shown in Figure _.

During the site visit, it appeared that the intersection may be hard to anticipate for motorists travelling on Great South Road. Vehicles waiting at the intersection on Mt. Richmond Drive and Bell Avenue were not clearly visible from both directions on Great South Road. Signs have been installed on Great South Road approximately 100m from the southern and northern ends of the intersection warning motorists of the cross intersection ahead. However the sign on the northern end is barely visible to motorists as it is covered by tree branches. There is also a speed sign displayed on Great South Road on the southern end displaying the 50km/hr speed limit. However the majority of the vehicles on Great South Road were observed to be approaching the intersection at speeds greater than 60km/hr. Trees, fence and temporary real estate signs on southern side of Bell Avenue are obstructing view of Great South Road for vehicles at the intersection. No clear view of traffic on GS Rd when approaching intersection on Bell Ave. Although there is Stop sign, most vehicles observed treat it as a Give Way sign.A courier depot belonging to PBT Couriers is situated on Bell Avenue. During the site visit, it was observed that a large proportion of vehicles turning from or into Bell Avenue were large trucks or vans belonging to PBT Couriers.

2.0 Accident HistoryIn the past 5 years, there has been 3 serious injury crashes with 5 casualties, 6 minor injury crashes with 11 casualties and 19 non-injury crashes as shown in the factor grid report (Appendix X). It is observed that 86% of those accidents including all the serious crashes happened at the intersection and 14% of crashes occurred on the straight road near the intersection (Appendix X-crash diagram). Majority of crashes included two common crash factors, which are failing to give way at stop sign, and poor observation. Crashes solely due to human factors such as fatigue and excess alcohol are excluded in this study and not counted in this report.

Further inspection showed that out of 9 injury crashes, 5 crashes occurred due to the car travelling eastbound on Bell Avenue crossing at right angle and failing to give way at uncontrolled intersection. Other two crashes were caused by the same reason, but from westbound on Mt Richmond Drive. The non-injury crashes are very closely related to this particular crash factor as well.

It is obvious that the main problem with this cross-intersection is the right turning vehicles from either Bell Avenue or Mount Richmond Drive onto Great South Road colliding with northbound and southbound traffic. This crossing-path crashes are mainly due to the result of the drivers inability to recognize and select a safe gap in the major street traffic stream especially at peak times where there are many turning vehicles from all directions. Also, when minor roadway volumes increase, the driver on the minor road will have more difficulty selecting a safe gap. Furthermore, heavy traffic at the uncontrolled intersection causes confusion for some drivers in terms of giving way, which can lead to serious head-on collisions.

During the site visit, it was noticed that there are intersection signs installed at both northbound and southbound ends of Great South Road (Approximately 100m from the intersection), but they are currently covered by overgrowing trees or shrubs, which may be a problem for drivers who arent familiar or new to the road. Also, the cross intersection appears to be hard to anticipate as clear view of traffic is not obtainable when travelling on Great South Road. This is backed up by the detailed crash records which shows that 50% of crashes involved poor observation as one of crash factors.

About 29% of crashes occurred when the road was wet and slippery and thus, it can be concluded that road surfacing and weather conditions werent major factor in the past accidents. The site visit was conducted under fine weather conditions and the pavement seemed relatively good condition. However, it is still recommended to improve pavement surface as it will increase friction and resistance to skidding and therefore, reduce accident rate in the future.

Crash type statistics from police reports and witness statements also classify 61% of all crashes as crossing/turning type followed by Rear End/Obstruction crashes being the second highest with 21%.

3.0 Improvement options and their benefits

OptionBenefit

Adjusting the speed limits to be appropriate for a given roadIncreases safe gap in the major street traffic stream

The use of porous pavement which reduces rear-end crashesBetter drainage during heavy rain makes the road much more skid resistant

The use of supplemental devices such as street light poles to mark the threshold between safe and unsafe lagsAllow drivers to merge into major traffic stream easier

Geometric improvements to reduce conflicts at intersection such as inside acceleration lanes (slip lanes), channelized median to eliminate certain maneuvers (Sometimes referred to as a J-Turn)Help delineate the area in which vehicles can operate, and to separate conflicting movements

Installing a traffic signal to assign right-of-way to the minor street Confusion at the intersection minimized as traffic control devices assign right of way

Major geometric improvements such as roundabout or grade separated interchanges to eliminate or reduce crossing conflictsGreater safety due to elimination of left turns, and also drivers tend to slow down when approaching a roundabout. Less stops and shorter queues result in reduced fuel usage

Transverse rumble stripsIncrease driver awareness that a dangerous intersection is ahead, and also encourage drivers to slow down

flush medianReduces the complexity of the turning/crossing by increasing visibility

widening roadsWidening road lines will give drivers more space to maneuver and improved safety conditions, ultimately leading to a reduction in the number of accidents.

Intersection with splitter islandDeflect and slow entering traffic, and also separate entering and exiting traffic. Increase driver awareness by channeling the traffic on the minor approaches

Based on the type of accidents from Collision Diagram and Crash Report (appendix x), 4 options are presented in this report consisting of different selection of elements as stated in Table x. The capital costs for each option have been calculated in Appendix x.

Option A - Do minimum

Option A is a Do Minimum scenario where a minimalist approach is taken to improve the intersection under consideration. This strategy option assumes maintaining current budget and levels of expenditure in new schemes. It is presumed that existing trends in travel behavior will continue. This option involves installing dynamic speed signs at both the northern and southern ends near the intersection advising vehicles to slow down if they are travelling over with the 50km/h as speed limit. suggested speed at both northbound and southbound ends of Great South Road, which will provide drivers a better insight on the approaching intersection. This will also encourage drivers to slow down prior to entering the intersection, providing drivers better insight as they approach the intersectoin. Slower speed around the intersection will also increase safe gaps for minor traffic to safely enter the major road. Secondly, porous pavement will be used to resurface the existing road. It makes the road much more skid resistant during and after heavy rain due to better drainage. Lastly, tTransverse rumble strips are installed to increase drivers awareness and to further slow down the speed. The reduction in accident costs were calculated by choosing the highest reduction and adding on X% of the sum of the reductions from dynamic signs and resurfacing. This is due to an unlikely result of X% reduction in total accident costs when all the reductions are added together individually. The main disadvantage of Option A is that crashes due to turning manoeuvres are not fully averted.

2 x slow down paintings on roads.

The total cost for Option A has been calculated in Appendix X.

Option B - Channelisation

Option B involves using channelisation to improve the intersection. Channelisation is achieved by installing pavement markings, delineators, splitter islands, and raised-medians which positively protect the lane from other traffic movements. Also, kerb-turning acceleration/deceleration lanes known as slip lanes will be installed for eastbound and westbound minor traffic. Channelisation will provide a separate path through an intersection for each traffic movement and thus, it will effectively remove a major source of confusion from the drivers mind and also make the drivers behaviour more predictable by other drivers. This option is expected to reduce crashes due to poor observation greatly as it provides clear vision of turning vehicles for the drivers at the intersection. Secondly, porous pavement will be used to resurface the existing road. It makes the road much more skid resistant during and after heavy rain due to better drainage

The main disadvantage of Option B is that channelisation can often be a problem for large vehicles due to minimum turning path. Also installing slip lanes would incur high cost of land acquisition.

The total cost for Option B has been calculated in Appendix X.

Option C - Traffic signalisationOption C involves installing vehicle actuated traffic lights at the intersection. The traffic lights will allow vehicles to cross or turn at the intersection in a more controlled manner than if the intersection was priority controlled. The signalisation of the intersection will eliminate driver error in decision making and driver confusion resulting from too many things happening at the intersection at once. Traffic signals will control all vehicles travelling straight ahead or turning right at the intersection, where as all left turns will be controlled by Give Way signs. The traffic signals will be vehicle actuated in order to minimise delays to traffic on Great South Road. This will mean that Great South Road traffic will be stopped only when vehicles are waiting on Bell Avenue and Mount Richmond Drive. The road on Great South Road will also need to be widened to allow for right turning lanes to allow vehicles from Great South Road to turn right onto Mount Richmond Drive and Bell Avenue without causing delays to traffic travelling straight ahead.

The main disadvantage of Option C is that signalisation will cause significant delays to vehicles on Great South Road.

Option D Widening Road and Installing Flush Median and Slip Lanes

4.0 Economic Evaluation of Improvements

BenefitsDo-MinimumNet Benefits (Do-Minimum Proposed Option)

Option AOption BOption COption DOption BOption COption D

(Do-Minimum)

Accident Cost $1,885,007.70$266,479.88$88,826.63$284,245.20$1,618,527.82$1,796,181.07$1,600,762.50

VOC $399,990.20$379,619.03$553,778.18$379,619.03$20,371.17-$153,787.98$20,371.17

Travel Time Cost$93,354.44$59,446.76$585,110.57$59,446.76$33,907.68-$491,756.13$33,907.68

Total Cost$2,378,352.34$705,545.66$1,227,715.38$723,310.99$1,672,806.68$1,150,636.96$1,655,041.35

Total Savings$3,345,613.35$2,301,273.92$3,310,082.70

PV of total net benefits$19,612,115$13,422,022$19,404,220

CostsCosts of OptionsNet Costs (Do-Minimum Proposed Option)

Option AOption BOption COption DOption BOption COption D

(Do-Minimum)

Capital Costs$16,000.00$300,000.00$700,000.00$200,000.00$284,000.00$684,000.00$184,000.00

Maintenance$1,000.00$5,000.00$10,000.00$5,000.00$4,000.00$9,000.00$4,000.00

Total$288,000.00$693,000.00$188,000.00

PV of net costs$330,809.20$789,320.70$230,809.20

FYRR672.30%165.51%831.50%

BCR59.4617.1584.32

Base option for comparisonNext higher cost optionCalculationIncremental BCRAbove/below the target incremental BCR (=4.0)

DB(19670626.59-19462731.43)/(330809.2-230809.2)2.0789516Below

DC(13539045.21-19462731.43)/(789329.7-230809.2)-10.60603186Below

The costs and benefits of the different options evaluated are presented in Table 1 and Table 2. The cost savings of Option B, Option C and Option D when compared with Option A, the Do-Minimum option, are also presented in Table 1. It can be seen that selecting Option B has the greatest net benefit of $19.6 million in comparison to the other options. The net benefits of Option D is close to Option B at $19.4 million, followed by Option C at $13.4 million. From Table 2, it can be seen that Option C will have the highest initial cost and maintenance cost over the next 30 years ($0.79 million), followed by Option B ($0.34 million) and then Option C ($0.79 million). Option D provides the greatest Benefits to Cost Ratio (BCR) of 84.32, where as Option Bs BCR is 59.46 and Option Cs BCR is 17.15. All of the options are likely to be approved for funding as the BCR of all options is greater than 4.0. Therefore, incremental analysis was performed and is presented in Table 3. From the incremental analysis, the incremental BCR for Option B and C, when compared with Option D, were less than the target incremental BCR of 4.0.

5.0 Conclusions and Recommendations

It is concluded that the main cause of intersection crashes results from right turning vehicles colliding with straight through traffic. The current intersection arrangement is a major problem due to the fact that it creates inadequate safe gaps for turning vehicles, which then ends up leading to head-on crashes. Other factors such as drivers awareness and visibility also play a major role in causing accidents.

Option D is considered to be the best option as it has a BCR of 84.32 which is far greater than option B and option C. The FYRR of option D is 831% and has net savings of $19.4 million. Option D was seen to be the most feasable option, as it has the highest BCR and will reduce the total accident cost for the site by $1.6 million. The accident cost savings from Option D is less than the savings expected from Option C of $1.8 million, however Option D is recommended as it has a significantly greater BCR of 84.32, compared with 17.15 for Option C. It is expected that by widening the road to accommodate a flush median while still keeping the capacity of Great South Road the same, the number of turning crashes at intersection will reduce significantly from 26 crashes to 20 crashes, including unreported crashes. These improvements are expected to reduce the complexity of the turning at the intersection and separate conflicting movements hence helping to decrease the number of accidents at peak times.

Appendix 1: CAS Report -Appendix 2: Do Minimum CalculationsAppendix 3: CAS Police Records Appendix 4: Detailed Site SummaryAppendix 5: Collion DiagramAppendix 6: Coded Crash ReportAppendix 7: Capital CostsAppendix 8: Cost of Option BAppendix 9: Cost of Option CAppendix 10: Cost of Option DAppendix 11: Accident CostsAppendix 12: Grid Factor DiagramAppendix 13: District Plan

Accident Cost Savings for Option D

Movement categoryVehicle involvement

1Do-minimum mean speed50Road categoryArterial

Posted speed limit50Traffic growth rate3%

2Option mean speed50

Do-minimumSeverity

FatalSeriousMinorNon-injury

3Number of years of typical accident rate records5

4Number of reported accidents over period03619

5Fatal/serious severity ratio (tables A6.19(a) to (c))0.080.9211

6Number of reported accidents adjusted by severity (4) (5)02.76619

7Accidents per year = (6)/(3)00.5521.23.8

8Adjustment factor for accident trend (table A6.1(a))0.93

9Adjusted accidents per year = (7) x (8)00.513361.1163.534

10Underreporting factors (tables A6.20(a) and (b))-1.52.87

10.1Percentage accident reduction due to Do-Minimum Improvements 10

11Total estimated accidents per year = (9) x (10) x (100 -(10.1)/100)00.6930362.8123222.2642

12Accident cost, 100km/h limit (tables A6.21(e) to (h))3800000405000240002400

13Accident cost, 50km/h limit (tables A6.21(a) to (d))3350000360000210002100

14Mean speed adjustment = ((1) 50)/501

15Cost per accident = (13) + (14) x [ (12) (13) ]3800000405000240002400

16Accident cost per year = (11) x (15)0280679.5867495.6853434.08

17Total cost of accidents per year (sum of columns in row (16)$401609.34

fatal + serious + minor + noninjury)

Option D - Flush Median and Widening Road

18Percentage accident reduction20202020

19Percentage of accidents remaining [ 100 (18) ]80808080

20Predicted accidents per year (11) x (19)00.55442882.24985617.81136

21Accident cost, 100km/h speed limit (tables A6.21(e) to (h))3800000405000240002400

22Accident cost, 50km/h speed limit (tables A6.21(a) to (d))3350000360000210002100





fatal + serious + minor + non-injury)

27Annual accident cost savings = (17) (26)$117364.14

28PV accident cost savings = (27) x DFAC$1260490.864E

Transfer PV of accident cost savings, E for the preferred option to E on worksheet 1.

Accident Cost Savings for Option C





Do-minimumSeverity



















Option C - Traffic Signalisation of Intersection














Accident Cost Savings for Option B





Do-minimumSeverity



















Option B - Channelisation














Capital and Maintenance CostsOption AOption A

ImprovementsCostMaintenanceCost/yr

2 x dynamic speed signs

4 x rumble strips

2 x replacement intersection signs

2x Slow down painting

Total cost $16,000 Total cost1000

Option BOption B


Pavement markings

Widening roads (2 extra lanes)

Raised-medians

Total cost $300,000 Total Cost5000

Option COption C


Traffic signals



Option DOption D


Flush median



Do Minimum CalculationsOption A - Do minimum

Major vehicle flow30729

Minor vehicle flow341

Major and minor speed45km/hr

Major Base travel time cost$16.27/h

Minor Base travel time cost$16.23/h

Delay

Major2.00sec/veh

Minor25.00sec/veh

Major additional travel time$0.30s/speed cycle

Minor additional travel time$14.60s/speed cycle

Traffic congestion cost

$3.20/hr

Travel time cost

Major

Speed change cycle$8,166.03

Queuing(stopped) time cost$54,440.18

Total sum$62,606.21

Minor

Speed change cycle$4,399.26

Queuing time(stopped) cost$7,532.97

Total sum$11,932.23

Traffic congestion cost$18,816.00

Total travel time cost$93,354.44

VOC

Length of the road0.1km

VOC by speed and gradident (50km/h, 0% gradient)24.6cents/km

Basic running cost$148,162.95

Roughness: assume 79 NAASRA0.3cents/km

Roughness cost$182,691.60

Speed change cost

Major road 50km/hr -> 45km/hr0.3cents/speed cycle

Minor road 50km/hr->0km/hr2.7cents/speed cycle

Speed change cost$19,873.22

Idle cost

Major road idle cost$3.18cents/min

Minor road idle cost$3.12cents/min

Idle cost $7,243.37

Traffic congestion cost6.9cents/km

-> volume 2163veh/hr, capacity of the road 2545veh/hr

congestion cost42019.068

Vehicle operating cost$399,990.20

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Civil461 - CAS Report