THE COBA 2017 USER MANUAL PART 2 THE VALUATION OF COSTS AND BENEFITS. Contents. Chapter. 1. The Valuation of Time Savings

Transcription

1 _ THE COBA 2017 UER MANUAL _ PART 2 THE VALUATION OF COT AND BENEFIT Contents Chapter 1. The Valuation of Time avings 2. The Valuation of Vehicle Operating Costs 3. The Valuation of Accidents 4. The Valuation of Accidents on Links 5. The Valuation of Accidents at Junctions 6. Construction Costs 7. The Preparation of Cost Data for Use in COBA 8. An Example of cheme Cost Inputs 9. Highway Maintenance 10. Delays During Construction 11. A ummary of the Items of Costs and Benefits 12. How to validate a COBA appraisal July 2017

2

3 Part 2 Chapter 1 The Valuation of Time avings 1. THE VALUATION OF TIME AVING 1.1 Changes in the time taken by traffic to pass along links and to pass through junctions in the COBA network are the major items of the calculated benefit resulting from a road improvement. It is necessary to put a money value on time savings in order to compare these with construction costs and accident and vehicle operating cost savings. 1.2 Three distinct purposes of travel are distinguished in COBA; travel in the course of Work, Commuting (travel to and from normal place of work) and Other (travel for other non-work purposes). The latest values of time recommended by the DfT for use in most routine economic appraisals of transport projects are provided in WebTAG data book. 1.4 Values of working time are estimated for different types of vehicle occupant and are given in Table 1/1. Buses include coaches; LGVs and OGVs are defined in Part 4 Chapter 5. OGV values apply to OGV1 and OGV2 classes as defined in Part 4 Chapter 5. For car drivers and passengers and bus passengers the value of working time is based on mileage-weighted incomes of a sample of workers to take account of the variation in mileage travelled by workers with different earnings. The data source for mileage weighting is the National Travel urvey. The information in Table 1/1 is taken from WebTAG data book but is presented in a form more suitable for use in COBA. 1.5 Deleted. 1.6 Deleted. 1.7 Deleted. 1.8 Deleted. 1.9 Deleted The COBA program has three vehicle purposes and three person purposes. Other goods vehicles (OGV) and their occupants are appraised in the working purpose only. Analysis of the National Travel urvey (NT) produces values for average vehicle occupancies and the proportion of car travel in working time. In combination with data from other sources it has also been possible to estimate the proportion of light goods vehicle travel not in working time (but not split between Commuting and Other non-work). Combining the values of time and occupancies produces values of time per vehicle (based on 2010 national average occupancies) as given in Table 1/1. July 2017 The COBA Manual 1/1

4 Chapter 1 Part 2 The Valuation of Time avings Type of Vehicle and Purpose Weekly Average Occupancy Working Car 1.00 driver Non-Working Car Commuting Other 0.16 passengers 1.00 driver 0.15 passengers 1.00 driver 0.91 passengers Average Car 1.00 driver 0.61 passengers Occupant Purpose Working Working (Ave working occupant) Commuting Commuting Other Non-Work Other Non-Work Value of time (pence/hour) per occupant per vehicle (1486) (Derived from above assuming 12.1% of car kilometres are in 'working' mode, 25.5% in 'commuting' mode and 62.5% in 'other' mode) Working Light Goods Vehicle (LGV) Non-Working LGV Commuting Other 1.00 driver 0.20 passengers 1.00 driver 0.59 passengers 1.00 driver 0.59 passengers Average LGV 1.00 driver 0.25 passengers Working Working Commuting Commuting Other Non-Work Other Non-Work (Derived from above assuming 88% of LGV kilometres are in 'working' mode, 2.6% in 'commuting' mode and 9.4% in 'other' mode) Other Goods Vehicle (OGV1&2) Public ervice Vehicle (PV) 1.00 driver Working driver Working passengers Working (1.5%) 842 (Ave working occupant) 872 Commuting (13.5%) 836 Other (85.0%) Average Vehicle (Based on 2002 national average vehicle proportions and 2010 occupancies) 1003 Table 1/1: Annual Average Values of Time per Person and per Vehicle in COBA Based on 2010 occupancies (2010 values and resource prices) 1.11 The NT provides data showing how car and LGV occupancies per kilometre travelled varies from hour to hour throughout the week. Table 1/2 shows how these vary by time of day. It is important to remember that because of differences in average trip lengths the occupancy per vehicle kilometre is different from occupancy per trip. 1/2 The COBA Manual July 2017

5 Part 2 Chapter 1 The Valuation of Time avings Flow Group /5 Weekday Weekend All W/day Ave /10 W/end Ave Working Car Week Ave Non-Work Car Commuting Other Average Car Working LGV Non-Work LGV Commuting Other Average LGV Table 1/2: Car and LGV Occupancies (per km travelled) by time of day (2010) 1.12 Car occupancy is assumed to remain unchanged as can be seen from Table 1/3. The occupancy of all other vehicle types also should be assumed to remain unchanged over time. Range of Years Vehicle Mode Rate of Change by Flow Group (% pa) 1 & & 5 6 & & onwards Working Cars Commuting Cars Other Cars Table 1/3: Compound Annual Rates of Change in Car Occupancies (%) 1.13 The National Travel urvey (NT) indicates how the proportion of car mileage in work time varies by hour of the week. Analysing this data with details of the allocation of hours to flow groups (see Part 4 Chapter 7) and car mileage showed how the proportion of car mileage in work time varies by flow group. The proportion defaults are given in Table 1/4. July 2017 The COBA Manual 1/3

6 Chapter 1 Part 2 The Valuation of Time avings Flow Group Work Time Proportion of flow in Commuting Other Non- Work Proportion of Annual Flow % 36.0% 52.0% 9.7% % 25.0% 56.5% 35.3% % 32.8% 51.2% 11.9% 4/5 14.7% 47.7% 37.6% 14.6% 6 5.3% 20.6% 74.1% 8.2% 7 2.1% 6.7% 91.2% 11.0% 8 3.1% 5.5 % 91.4% 4.1% 9/10 3.1% 5.5% 91.4% 5.2% Table 1/4: Proportion of Car Mileage by Mode by Flow Group 1.14 Local proportions of cars in work and non-work time can be used where there is statistically reliable local data. However in the majority of cases it is unlikely that local values will be significantly different from the national. When local data is available it will usually be from 12 hour or 16 hour roadside interview surveys and it will be necessary to convert these to an average weekday. Table 1/5 shows how the national work time proportion varies by time period. The local short period observation can be factored pro rata to obtain the local annual average value. Flow Group Average Weekday 7am 7pm Weekday Average Weekend Work Time Proportion of flow in Commuting Other Non- Work Average Day Table 1/5: Proportion of Car Mileage in Work Time by Time Period 1.15 The real value of average employee earnings is assumed to reflect the growth in the real value of both working time and non-working time. Future growth is expected to grow in line with real GDP per head. Forecasts of growth in the real value of time are given in Table 1/6 1/4 The COBA Manual July 2017

7 Part 2 Chapter 1 The Valuation of Time avings Year Work VOT Growth (% pa) Forecast Growth In Values of Time Non-work VOT Growth Year (% pa) Work VOT Growth (% pa) Non-work VOT Growth (% pa) onward Table 1/6: Assumed Compound Annual Rates of Growth of the Real Value of Time (% pa) These assumptions relate to long term forecasts only, and undue weight should not be given to short term fluctuations. The Calculation of Time User Costs in COBA 1.16 The stages by which COBA calculates time user costs for the links and junctions in a Do Minimum or Do- omething network are set out below: i) traffic flows are disaggregated by vehicle category, (CAR, LGVs etc.) based on the flow of vehicles in each hour (by flow group - see Part 4 Chapter 8); ii) the flow is further disaggregated by journey purpose, that is, 'work', 'commuting' and 'other non-work'. OGV trips are all assumed to be in work, PVs are work vehicles with a non-work passenger element, cars are mainly 'other non-work' (52%); iii) the hourly flow of vehicles for each link is thus defined by vehicle category, journey purpose, flow July 2017 The COBA Manual 1/5

8 Chapter 1 Part 2 The Valuation of Time avings group and year; iv) the time cost per vehicle is a function of the number of occupants (which varies over time for 'other non-work' cars) and journey purpose; v) the hourly flow of vehicles is converted to an hourly flow of people by multiplying the number of vehicles by the appropriate occupancy rate; vi) COBA then calculates time costs by multiplying the flow of people by the value of their time. This applies to link transit and junction delay time costs. Conversion from Resource Costs to Market Prices 1.17 COBA works in resource costs that have to be converted to market prices for presentation in the Transport Economic Efficiency (TEE) Table. The market price of time is obtained by multiplying the resource value by (1 + t) where t is the average rate of indirect taxation in the economy; the current indirect taxation rate is 19% 1/6 The COBA Manual July 2017

9 Part 2 Chapter 2 The Valuation of Vehicle Operating Costs 2. THE VALUATION OF VEHICLE OPERATING COT 2.1 Differences in the vehicle operating costs (VOC) incurred by traffic using the Do-omething road network compared to the VOC incurred by traffic using the Do-Minimum network are recorded among the benefits resulting from a road improvement. 2.2 The change in total VOC over all links depends on changes in the distance travelled by vehicles and on average link speeds. In most schemes the aggregate time saving is positive, as is the overall saving in costs of accidents, but the change in overall VOC can be either negative or positive depending on the balance of changes in distance travelled and speeds. The latest vehicle operating cost (VOC) parameter values recommended by the DfT for use in economic appraisals of transport projects are provided in WebTAG data book. 2.3 VOC in COBA comprises six items: fuel (petrol / diesel fuel, or, at present in the case of cars only, mains electricity), oil, maintenance, depreciation, and size of vehicle fleets. Only items which vary with the use of the vehicle are measured, so, for example, vehicle excise duty, insurance, and garaging are excluded from VOC. More precisely it is the mileage related resource VOC that is included in COBA. All resource costs exclude any element of indirect taxation as explained in Chapter 6 of Part 1. Fuel Consumption 2.4 Fuel consumption is estimated using a function of the form: L = a/v + b + c V + d V 2 Where L = consumption, expressed in litres per kilometre V = average link speed in kilometres per hour, a, b, c and d are parameters defined for each vehicle category. 2.5 This fuel consumption is then converted into a resource cost pence per kilometre by multiplying by the cost of fuel. 2.6 There is no specific allowance in COBA for fuel used at junctions. Non-Fuel VOC 2.7 The non-fuel elements of the marginal resource cost are combined in a formula of the form: C = a 1 + b 1 /V, where C, V, a and b are defined as in paragraph The marginal resource costs of oil, tyres, mileage and maintenance related depreciation are assumed to be fixed costs per kilometre and appear in the a term. The b term in the non-fuel costs represents changes in the productivity of commercial vehicles and cars in working time, all goods vehicles and PVs. 2.9 The time component of depreciation is excluded since it does not vary with distance or speed. For OGVs and PVs depreciation is assumed to be totally time related; this is based on evidence from trade sources which July 2017 The COBA Manual 2/1

10 Chapter 2 Part 2 The Valuation of Vehicle Operating Costs suggests that factors such as obsolescence and condition are more important determinants of vehicle value than mileage per se. For cars and LGVs evidence from second hand prices indicates that part of their depreciation is related to mileage; therefore this element is recorded as a marginal resource cost. VOC calculation parameters 2.10 The parameters needed to calculate the fuel consumption elements of VOCs are given in Table 2/1. The fuel consumption parameter values are based on a 2010 vehicle fleet (updated from the 2010 values in previous versions), whilst the electrical energy consumption values are based on 2011 values. Parameters Vehicle Category a b c d Fuel Consumption Parameter Values (litres per km, 2010) Petrol Car E E-06 Diesel Car E-06 Petrol LGV E E-06 Diesel LGV E-06 OGV E-05 OGV E-05 PV E-05 Energy Consumption Parameter Values (kwh per km, 2011) Electric Car Table 2/1: VOC Formulae 2010 Parameter Values in litres/km 2.11 In Table 2/1a, the VOC parameters have been converted into pence per kilometre by multiplying by the cost of fuel, shown in Tables 2/1b. Vehicle types OGV1, OGV2 and PV are assumed to be diesel driven and therefore parameters for these vehicles have been multiplied by the resource cost of diesel. The parameters for average car and average LGV are calculated as the average across different fuel/energy types. In the absence of more specific evidence it has been assumed that the proportions of traffic by fuel type are broadly equivalent to the proportions of vehicle kilometres by fuel type as shown in Table 2/1c. 2/2 The COBA Manual July 2017

11 Part 2 Chapter 2 The Valuation of Vehicle Operating Costs Vehicle Category a b c d Values excluding VAT (for vehicles in course of work) Petrol Car Diesel Car Average Car Petrol LGV Diesel LGV Average LGV OGV1 (diesel) OGV2 (diesel) PV (diesel) Values including VAT (for vehicles in course of other purposes) Petrol Car Diesel Car Average Car Petrol LGV Diesel LGV Average LGV Table 2/1a: VOC Formulae 2010 Parameter Values in pence/km July 2017 The COBA Manual 2/3

12 Chapter 2 Part 2 The Valuation of Vehicle Operating Costs Year Resource Cost Duty VAT rate Petro l Diese l Gas Oil Electricity Road Rail Petro l Diese l (p/litre) (p/litre) (p/litre) (p/kwh) (p/kwh) (p/litre) (p/litre) Gas Oil (p/litre) Electri c (p/kwh) Petro l (%) Diese l 2002* * * * * * * * * * * *actual prices / rates Table 2/1b. Fuel and Electricity Prices and Components (2010 prices) (%) Electri c Road (%) 2/4 The COBA Manual July 2017

13 Part 2 Chapter 2 The Valuation of Vehicle Operating Costs Year Cars LGVs Petrol Diesel Electri c Petrol Diesel % 26.90% 0.00% 8.50% 91.50% % 40.00% 0.00% 3.68% 96.32% % 50.13% 0.15% 2.16% 97.74% % 54.18% 0.94% 1.34% 98.43% % 51.99% 3.50% 1.09% 97.16% % 47.78% 8.17% 0.82% 93.32% Table 2/1c: Proportion of Cars and LGV Vehicle Kms Using Petrol, Diesel or mains electricity 2.12 COBA adjusts the values of the fuel cost parameters a, b and c over time. This reflects changes in the price of fuel and also the fuel efficiency of vehicles. For cars it also reflects the changes in the proportion of the vehicle fleet using either petrol or diesel. The annual changes in fuel resource costs per vehicle kilometre and in the proportion of the car fuel usage are given in Tables 2/1b and 2/1c, respectively. The annual percentage changes in fuel usage per kilometre due to fuel efficiency improvement are given in Table 2/2. Year Petrol Car (actual) (actual) (actual) (actual) Vehicle Fuel Efficiency Improvement Diesel Electri Petrol Diesel OGV1 OGV2 PV Car c Car LGV LGV (actual) (actual) (actual) (actual) (actual) (actual) (actual) (actual) Table 2/2 Vehicle Fuel Efficiencies Conversion from Resource Costs to Market Prices 2.13 COBA works in resource costs that need to be converted to market prices for inclusion in the Transport July 2017 The COBA Manual 2/5

14 Chapter 2 Part 2 The Valuation of Vehicle Operating Costs Economic Efficiency (TEE) Table. Because the resource cost of fuel, fuel efficiency and fleet composition change independently the relationship of resource cost per kilometre to market price changes annually. Consequently the program needs to be informed of the changes in tax rates over time. The 2010 base Fuel Tax Rates (that is, the tax expressed as a proportion of the resource cost) and the predicted changes over time are included in Tables 2/1b and 2/1c The intermediate tax rate is used to convert the resource cost to the price excluding VAT and the final tax rate converts to the price including VAT. For example, the cost including VAT equals the resource cost multiplied by (1 + final tax rate) The tax rates for non-fuel VOC are assumed to be constant over time as they are only affected by VAT. The VAT rate is assumed to remain constant over time. Carbon Dioxide Emissions 2.16 Carbon dioxide emissions should be considered in terms of the change in the equivalent tonnes of carbon dioxide released as a result of implementing a highway scheme, see WebTAG Unit A3 for further details. Carbon dioxide emissions are estimated from fuel consumption in the Do Minimum and the Do omething options. The change in tonnes of carbon dioxide emitted and the monetary value given to the change has been added to COBA. The emission rates from WebTAG Unit A3 used in COBA are listed in Table 2/3 while the costs per tonne of carbon dioxide are given in Table 2/4. Table 2/4 lists three different cost assumptions (low, central, high); only the central assumption is used to calculate the final Emissions Benefit figure in Table 15C. However, results based on all three assumptions are included in the Table 14 output. 2/6 The COBA Manual July 2017

15 Part 2 Chapter 2 The Valuation of Vehicle Operating Costs Year Road Rail Petrol Diesel Electricity Gas oil Electricity kgco 2 e/ kgco 2 e/ kgco 2 e/kw kgco 2 e/ kgco 2 e/kw l l onwards Table 2/3 Carbon Dioxide Emissions per litre of Burnt Fuel h l h July 2017 The COBA Manual 2/7

16 Chapter 2 Part 2 The Valuation of Vehicle Operating Costs Low Central High Low Central High Table 2/4 Cost per Tonne of Non Traded Carbon Dioxide 2/8 The COBA Manual July 2017

17 Part 2 Chapter 3 The Valuation of Accidents 3. THE VALUATION OF ACCIDENT 3.1 The benefits from a reduction in the number and severity of accidents constitute an important element in the appraisal of trunk road schemes. It is necessary to put a money value on accident savings so that they are given an appropriate valuation relative to that given to construction costs and to time and vehicle operating cost savings. tandard values are produced for use in valuing the savings of accidents resulting from road improvements and road safety measures. The values used in COBA are derived on the same basis as given in WebTAG Unit A4.1. Table 3/1 details the components of accident costs which in addition to the casualty cost include the costs associated with damage to property, insurance administration, police time and an allowance for damage only accidents. COT PER CAUALTY, Fatal casualty erious casualty light casualty 1,556, ,878 13,481 COT PER ACCIDENT, Fatal accident erious accident light accident Damage only Insurance Administration Damage to Property Urban Rural Motorway Police Cost Urban Rural Motorway Fatal accident erious accident light accident Damage only Number of Damage Only Accidents per pia Table 3/1: Components of Accident Costs (2010 values and prices) 3.2 To determine overall accident costs, details of the average accident severity split (that is, the number of fatal, serious and slight casualties per accident) and the proportion of fatal/serious/slight accidents are required. Information on the severity split by links and junctions type is given in Part 2 Chapter 4 for links and Chapter 5 for junctions. Note that accidents are classified according to the most seriously injured casualty and that for accident coding purposes rural roads are defined as those with a speed limit of more than 40 mph (64 kph). Those with speed limits of 40 mph or below are defined as urban roads. July 2017 The COBA Manual 3/1

18 Chapter 3 Part 2 The Valuation of Accidents 3.3 Details of the average proportion of fatal/serious/slight accidents on links are given in Table 3/2 and at junctions in Table 3/3. The proportions are based on data. Accident rates and severity have been reducing over recent years and this trend is expected to continue into the future; see paragraph 4.5 and Table 4/3 in Chapter 4. The forecasting of the proportion of fatal/serious/slight accidents is based on a similar methodology. First, the fatal and serious proportion is forecast by applying the accident rate change coefficient given in Table 4/1 in Chapter 4. The sum of these is subtracted from unity to determine slight proportion. LINK ONLY ACCIDENT PROPORTION (2009 Base) ACCIDENT TYPE ROAD TYPE ACCIDENT PROPORTION Casualty everity Fatal (f) erious (se) light (sl) 1 3 Motorways peed Limit 30/40 mph > 40 mph Casualty everity f se sl f se sl A Roads Other 2 Roads Dual Carriageways LINK AND JUNCTION COMBINED ACCIDENT PROPORTION (2009 Base) Casualty everity Fatal (f) erious (se) light (sl) 1 3 Motorways peed Limit 30/40 mph > 40 mph Casualty everity f se sl f se sl A Roads Other 2 Roads Dual Carriageways Table 3/2: Proportions of Fatal, erious and light Accidents on Links (Average ) 3/2 The COBA Manual July 2017

19 Part 2 Chapter 3 The Valuation of Accidents PROPORTION OF ACCIDENT AT JUNCTION (2000 Base) peed Limit 30/40 mph > 40 mph Casualty everity f se sl f se sl 3 Arm Priority and 5 arm Priority Arm Traffic ignals and 5 Arm Traffic ignals Roundabouts (tandard and mall) Mini Roundabout ignalised Roundabouts Table 3/3: Proportions of Fatal, erious and light Accidents at Junctions (Average ) 3.4 The average accident costs used in COBA will normally be appropriate even where local accident rates differ from the average. In some circumstances the severity split may differ with a consequent change in average accident costs, but this is only likely to be significant in a few cases. The Overseeing Organisation should be consulted if it is considered necessary to use local severity splits. General policy is to discourage the use of a local severity split unless it can be shown that exceptional conditions genuinely arise which are unlikely to be corrected by modest remedial works. 3.5 In order to accept a local severity split the user must: i) demonstrate that the severity split is significantly different in statistical terms from the COBA value, and also that this does not result from one or two particularly bad accidents, the effect of which will be evened out by less extreme accidents as time goes by. Data covering all available accident history, with a minimum of five years must be supplied; ii) arrange an Accident Investigation and Prevention tudy by the Local Authority to identify the causes of the safety problem and recommend remedial safety measures. Where this study concludes that modest remedial works are unlikely to correct the problem then a local severity split may be used. However where modest remedial works are recommended, the cost of these works should be included in the "do minimum" and the revised COBA severity split used. 3.6 tatistics of damage only accidents are not generally available because they are not comprehensively reported by the police. On the basis of some survey information, these are taken in COBA to occur at the rates given in Table 3/1. These rates are assumed to remain constant over time. The accident costs given in Part 2 Chapter 4 and Chapter 5 include an allowance for damage only accidents at these rates. 3.7 For the purposes of appraisal it is necessary to form a view on how costs will vary over future years. The value of most elements of accident costs are proportional to national income, and for this reason it is assumed that values change in line with GDP per head. The actual and forecast changes in GDP per head are given in Table 3/4. July 2017 The COBA Manual 3/3

20 Chapter 3 Part 2 The Valuation of Accidents Year Change in Value of Accident (% pa) Year Change in Value of Accident (% pa) onward 2.06 Table 3/4: Assumed Compound Annual Rates of Growth of Accident Values (%) 3.8 The total cost of accidents on a road network is calculated by multiplying the number of accidents predicted to occur on the network by the cost per accident. As explained above, the cost per accident varies by type and area of road. The number of accidents on a given length of road is expressed as an accident rate, defined as so many Personal Injury Accidents per million vehicle kilometres, so that doubling either the length or the traffic flow on the road will double the number of accidents. Apart from length and flow level, in COBA there are two determinants of the number of accidents: the number and type of junctions and the type of links. 3.9 COBA incorporates a method of separating out the effects of links and junctions on accidents. Where junctions are coded for delay calculation, these should be coded for accident calculation. In addition, where there are junctions which are subsumed in links for speed calculations (in particular in urban areas), but which are likely to be associated with accidents, these should be coded as accident-only nodes. Finally, where either a very large link-only network is used and accident-only nodes are difficult to identify, or local data on existing accidents are difficult to split between links and junctions, combined link and junction accident rates can be attributed to links. The treatment of accidents on links and junctions is described in detail in the 3/4 The COBA Manual July 2017

21 Part 2 Chapter 3 The Valuation of Accidents following two Chapters. July 2017 The COBA Manual 3/5

22 Chapter 3 Part 2 The Valuation of Accidents 3/6 The COBA Manual July 2017

23 Part 2 Chapter 4 The Valuation of Accidents on Links 4. THE VALUATION OF ACCIDENT ON LINK 4.1 The COBA user has to decide whether to code junction accidents separately from link accidents. Combined accident rates and costs attribute all accidents to links. The link-only rates and costs exclude junction accidents (that is, those occurring within 20 metres of a junction) and so both default and local link-only accident rates are lower than the combined rates for a particular link. Accidents at junctions are then modelled separately (see Part 2 Chapter 5). The preferred method of evaluating accidents is to separate link and junction accidents, using local accident data to define the Do-Minimum rates and the default rates for new links and junctions in the Do-omething. However, Combined accident rates and costs should be selected in COBA when: 4.2 Deleted. i) local data for the Do-Minimum are not available; ii) local data have already been collected in combined form and resources to disaggregate the data are not available; or, iii) a large network is being used and accident-only nodes would be difficult to identify. 4.3 Local data can be obtained from the appropriate police or local authority and should relate to a period when conditions on the road have been broadly unchanged (for example, no abnormal changes in traffic flow, no changes in junction design or road geometry, etc). Local data should normally cover the five years previous to the COBA assessment and must cover at least three years. The number of accidents in each year must be input, including zero for those links or years where no accidents occurred, and COBA will then internally produce a local accident rate (accidents per million vehicle kilometres) for each link. 4.4 For existing links where local accident data are not available and for new links, the program will use the default accident rates shown in Table 4/1, they are based on data from There are fifteen accident types relating to the type of road which are further subdivided by speed limit. In the following tables many cells in the table are at present identical; when available data permit separate rates to be calculated users will be advised. The Modern, Older and Other link descriptions used in the tables are defined as: Modern roads designed and built to geometric standards relevant post 1980, Older the majority of the major road network which was not built to recent standards. For single carriageways this description refers to A roads only, and Other B, C and unclassified single carriageway links. 4.5 Accident rates and severities have been falling steadily over time and the trend is expected to continue in the future. The declining trend in accident rates was examined in TRL Report 382 and at a more disaggregate level in later work undertaken for the DETR. It was found that the changes in accident rates and the number of severities per accident are explained by the relationship: A N = A 0 x b N Where: A N = the accident rate or number of casualties per accident N years after base year; A 0 b N = the accident rate or number of casualties per accident in the base year; = change coefficient raised to the power N (the number of years after the base year). 4.6 The values for the accident rate change coefficient b incorporated in the COBA program for the different link accident types are given in Table 4/1. They are the same for Link Only and Link and Junction Combined analyses and should be applied from any year from 2004 until year Between 2020 and 2029 and 2030 and 2039 the accident rate change is assumed to be one half and one quarter respectively of the 2004 to 2019 July 2017 The COBA Manual 4/1

24 Chapter 4 Part 2 The Valuation of Accidents on Links reduction. For example, if the coefficient b is 0.9 for 2004 to 2019 then it is 0.95 for 2020 to 2029 ( or [1 + b]/2 ). Zero change is assumed post ACCIDENT TYPE ROAD TYPE D2 Motorway D3 Motorway D4 Motorway Modern 2 Roads Modern 2 Roads with H Modern W2 Roads Modern W2 Roads with H Older 2 A Roads Other 2 Roads Modern D2 Roads Modern D2 Roads with H Older D2 Roads LINK ONLY (2009 Base) Accident Rate (Pia/mvkm) * * * b /40 mph 50/60/70 mph Pia/mvkm b Pia/mvkm b Modern D3+ Roads Modern D3+ Roads with H Older D3+ Roads ACCIDENT TYPE LINK AND JUNCTION COMBINED (2009 Base) ROAD TYPE D2 Motorway D3 Motorway D4 Motorway Modern 2 Roads Modern 2 Roads with H Modern W2 Roads Modern W2 Roads with H Older 2 A Roads Other 2 Roads Modern D2 Roads Modern D2 Roads with H Older D2 Roads Modern D3+ Roads Modern D3+ Roads with H Older D3+ Roads Accident Rate (Pia/mvkm) ** ** ** b /40 mph 50/60/70 mph Pia/mvkm b Pia/mvkm b Notes: H refers to the one metre wide hard strip provided both sides of the carriageway; * this rate includes accidents at merge/diverge areas and on slip roads; ** this rate includes accidents at the ends of the slip roads, for example, at roundabout, traffic signals etc. 4/2 The COBA Manual July 2017

25 Part 2 Chapter 4 The Valuation of Accidents on Links Table 4/1: Default Accident Rates and Accident Rate Reduction Factor (b) (personal injury accidents per million vehicle kilometres Base) 4.7 Table 4/2 shows the severity split incorporated in COBA in terms of casualties per accident by link type based on data for LINK ONLY CAUALTIE (2009 Base) ACCIDENT TYPE ROAD TYPE CAUALTIE PER P.I.A. Casualty everity Fatal (f) erious (se) light (sl) 1 3 Motorways peed Limit 30/40 mph > 40 mph Casualty everity f se sl f se sl A Roads Other 2 Roads Dual Carriageways LINK AND JUNCTION COMBINED CAUALTIE (2009 Base) Casualty everity Fatal (f) erious (se) light (sl) 1 3 Motorways peed Limit 30/40 mph > 40 mph Casualty everity f se sl f se sl A Roads Other 2 Roads Dual Carriageways Table 4/2: Average Number of Casualties per Accident (2009 Base) July 2017 The COBA Manual 4/3

26 Chapter 4 Part 2 The Valuation of Accidents on Links 4.8 Table 4/3 gives the casualty rate reduction factors b for each link type incorporated in the COBA program. The changes are assumed to apply up to 2019 with zero change thereafter. LINK ONLY Change Factors b ACCIDENT TYPE ROAD TYPE ALL PEED LIMIT Casualty everity Fatal (f) erious (se) light (sl) 1 3 Motorways peed Limit (mph) 30/40 mph > 40 mph Casualty everity f se sl f se sl A Roads Other 2 Roads Dual Carriageways LINK AND JUNCTION COMBINED Change Factors b ACCIDENT TYPE ROAD TYPE ALL PEED LIMIT Casualty everity Fatal (f) erious (se) light (sl) 1 3 Motorways peed Limit (mph) 30/40 mph > 40 mph Casualty everity f se sl f se sl A Roads Other 2 Roads Dual Carriageways Table 4/3: Casualties Per Accident Change Factors b. 4.9 Table 4/4 shows the accident costs incorporated in COBA by link type using the average severity splits given in Table 4/2. The severity split and hence cost varies considerably by link type. Junction accidents are, on average, less severe than those attributable to links and therefore the combined link and junction cost is lower than the link-only average. Because the severity of accidents is expected to reduce over time the average accident costs change each year and the costs in the table are only attributable to the Base year of the data. 4/4 The COBA Manual July 2017

27 Part 2 Chapter 4 The Valuation of Accidents on Links LINK ONLY COT (2009 Base) ACCIDENT TYPE ROAD TYPE ACCIDENT COT ( ) 1-3 Motorways 102,851 peed Limit 30/40 mph 50/60/70 mph A Roads 102, ,384 9 Other 2 Roads 92, , Dual Carriageways 104, ,260 LINK AND JUNCTION COMBINED COT (2009 Base) 1-3 Motorways 100,080 peed Limit 30/40 mph 50/60/70 mph A Roads 87, ,585 9 Other 2 Roads 84, , Dual Carriageways 85, ,598 Table 4/4: Average Cost per Injury Accident by Link Type (2009 Base) (2010 values and prices) July 2017 The COBA Manual 4/5

28 Chapter 4 Part 2 The Valuation of Accidents on Links 4/6 The COBA Manual July 2017

29 Part 2 Chapter 5 The Valuation of Accidents at Junctions 5. THE VALUATION OF ACCIDENT AT JUNCTION 5.1 COBA can estimate the numbers and costs of accidents at junctions and on links separately. This is the recommended method for COBA appraisal and should be used unless information on junction characteristics is lacking. All new junctions should be coded for accident appraisal, as should all existing junctions where there are significant forecast traffic flow changes and where accidents are likely to occur; junctions which are coded for accident but not traffic delay purposes are called accident-only nodes. In the absence of local data on junction-attributable accidents COBA default values for junctions should be used. 5.2 COBA incorporates two models that relate accidents at junctions to given flow configurations. These allow forecasts of future accident numbers to be derived for existing and new junctions. For existing junctions, the use of local accident data is recommended, with default values being used in the absence of such data. The number of accidents occurring at (that is within 20 metres of) each junction as recorded by the appropriate police or local authority should be used. Local data should normally cover the five years previous to the COBA assessment and must cover at least three years. 5.3 The models are of two types, both of the basic form: A = a (f) b, where A is the annual number of accidents, f is a function of traffic flow and a and b vary among junction types. They have been estimated by reference to accidents and flows at existing junctions. The choice of function varies according to junction type as indicated in Table 5/1 which also shows the associated values of a and b. 5.4 In the Cross Product (C) model, (f) is the value produced by multiplying the combined inflow from the two major opposing links by the sum of the inflows on the other one or two minor links. Inflows are measured in thousands of vehicles per annual average day. In the inflow (I) model, (f) is the value of the total inflow from all links in thousands of vehicles per annual average day. Where the user inputs local accident numbers for existing junctions, the COBA program calculates a local value for a, with b being fixed at the national value. 5.5 Deleted. 5.6 The classification of junctions for accident assessment distinguishes three broad categories, that is, major/minor, signals and roundabouts (subdivided into standard, small, mini and signalled). Major/minor junctions include staggered, standard priority, multiple, Y-junctions and cross-roads. taggered junctions can be treated either as crossroads or as pairs of three-arm junctions. Normally, where the stagger is significant and the junction effectively operates as two T-junctions, it should be coded as two three-arm junctions. This also applies to the coding of junctions for delay purposes (see Part 6 paragraph 8.22). July 2017 The COBA Manual 5/1

30 Chapter 5 Part 2 The Valuation of Accidents at Junctions 5/2 The COBA Manual July 2017 Junction Type No. of Arms Highest Link tandard (ingle or Dual) Formula Type NON BUILT-UP (Above 40 MPH) BUILT-UP (UP to 40 MPH) Junction Type Coeff a Power b Junction Type Coeff a Power b PRIORITY D D D C C I C I I PRIORITY WITH GHOT ILAND D D D C C I C I I PRIORITY WITH INGLE LANE DUALLING D D D C C I C I I IGNAL /6 5/6 D D D I C C C I I ROUNDABOUT TANDARD /6 5/6 D D D C C C C I I MALL /6 5/6 D D D C C C C I I MINI /6 5/6 D D D C C C C I I IGNALLED /6 5/6 D D D C C C C I I Table 5/1: Junction Accident Parameters 1997 Base

31 Part 2 Chapter 5 The Valuation of Accidents at Junctions 5.7 As an example, imagine a rural 4-arm major/minor junction with inflows as shown in Figure 5/1 (AADT in '000s): Figure 5/1: Example of 4-Arm Major Minor Junction 10 The major road in this case is a dual carriageway and so the relevant formula is for junction type 7, is of the Cross Product type, and is: A = 0.24 (f) Given the above flows, the value of the flow factor (f) is 54 (that is, (8+10) x (1+2)) and the predicted number of personal injury accidents per annum is: A = 0.24 (54) = In all applications of the Cross Product model, any combined inflow from the one or two minor opposing links that amounts to less than 1 (that is, 1000 AADT) will be taken by COBA to be 1 because such low flows were rarely encountered in the research which produced the accident formulae; consequently little evidence of the effect of changes in very low combined inflows was found. In such cases the formulae are sensitive only to changes in combined minor or major link inflows which involve inflow levels over 1000 (AADT). This cut-off applies also to the combined inflow from the two major flows. However it will usually be inappropriate to model such low flow junctions as paragraph 5.6 and Table 5/2 make clear. If the junction had single carriageways only, the formula would be of the Inflow type for junction type number 5. Where the value of the flow factor (f) is 21 (that is, ); the formula would be: A = (21) = It must be stressed that these junction accident formulae have been derived from actual records of accidents and flows at junctions. The accident predictions for given junction types with flow levels and configurations outside the ranges recorded in the research are not a reliable guide to design, although they provide useful indications when used in conjunction with other methods of operational analysis. They should be used with great care when considering individual junctions, for example, junction accident rates may vary to some extent with the local spacing of junctions and other characteristics not included in the COBA accident formulae. When comparing the COBA accident benefits of different junction types (as part of an economic appraisal of preferred junction type), the user should check that the ranking by accident benefits accords with engineering July 2017 The COBA Manual 5/3