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Road Design

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Design standard Design standards refers to strategic

decisions regarding the geometric standard to which the road is build

These decisions usually made at planning stage and primarily affected by capacity and economic efficiency considerations

Safety should also be a consideration in planning phase

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Road standard As traffic flow increases it becomes

necessary or economical to design and build roads to higher economic standard

The higher the economic standard, the safer the road

Safety benefits of higher geometric standards is one of the economic factors that should be taken to account in deciding appropriate design standard

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Road standard

Investigating safety benefits of road improvement show the following crash reduction significant at 10 percent level or better for various types of projects Rural town bypasses : 32% reduction Rural road duplication : 29% reduction Urban intersection grade separation : 57%

percent Other rural projects : 28% reduction

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Road standard The highest geometric of road is one with

High design speed Full access control Forgiving roadsides Entry and exit at grade-separated

interchanges Opposing direction of traffic separated by a

median This type of road is variously referred to as

freeway, motorway, autobahn, and sometimes expressway

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Road standard Safety improves dramatically with design

standard Freeways are much safer per mile of

travel than other roads Freeways with higher standards are

at lest 4 times safer than other roads 20 times safer than other arterial rods 2 times safer than freeways with lower

standards

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Access control Access control reduces or eliminates

the events which the driver must respond

Access control has been described as the most important single design factor ever developed for crash reduction

Part of safety advantages of freeways stem from control of access

Controlling access on existing road through the use of frontage roads

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Access control (continued) Crash rates increases rapidly with the

density of access driveways In one study the difference between a low

development (fewer than 30 access per mile) and a high level of development was to more than double the number of driveway crashes

Several studies indicated that this is a rural problem as well as an urban problem in US

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Median Medians are of several types

Wide median without a physical barrier• Provide space for the driver to regain control and

provide room for turning lanes Narrow median with a physical barrier

• Steel guard fence or shaped concrete barrier• Discouraging inappropriate pedestrian crossing

Narrow median without a physical barrier• Separating opposing direction of traffic• Provide opportunity for pedestrians to cross the

road in two stages

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Median (continued) A study by National Association of

Australian state road compared crash rate for roads with different median types and compared with undivided roads Narrow painted median

• 30% crash reduction Narrow raised median

• 48% crash reduction Wide median

• 54% crash reduction

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Median (continued) In urban areas, medians should be wide

enough to protect a turning or crossing vehicles

FHWA in 1982 found that with a 30 feet wide median, between 70-90 percent of vehicle encroaching on the median do not reach the other side of roadway

Narrower medians with physical barrier typically have higher crash rate but lower crash severity

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Median (continued) A British study in 1980 found that installing a

steel guard fence in the median of divided rural roads led to 15% reduction in fatal crashes 14% increase in non-injury crashes

The median slope on wide medians can influence crashes Zegeer and Council 1992 suggest that

• a maximum slope of 6:1 is desirable on wide medians• Slopes of 4:1 or steeper are associated with rollovers

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Cross section elements (continued)

Lane width 11-12 ft lanes have been shown to have the

lowest crash rate 10 ft lanes have been shown to contribute

to multi-vehicle crashes A number of studies have shown the safety

advantages of widening narrow lanes• An American study reveal 22% crash reduction

on rural roads by widening 9 ft lanes to 11 ft and 10 ft lanes to 12 ft

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Cross section elements (continued)

Widening lane width affected opposite direction and run-off-road crashes by following percentages 1 ft lane widening : 12% crash reduction 2 ft lane widening : 23% crash reduction 3 ft lane widening : 32% crash reduction 4 ft lane widening : 40% crash reduction

Study shows that there is little if any benefit in increasing lane width beyond 12 ft, except where there is a large volume of trucks, where lanes of 13 ft may be appropriate

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Cross section elements (continued)

Wider lanes may be counter-productive, since they encourage unsafe maneuvers such as overtaking along center line in the face of oncoming traffic

Striping the road with three lanes Better from safety and user service viewpoints Clearly define overtaking lane n one direction Much cheaper, since overtaking lane need not

to be provided over full length of the road Overtaking lane in about 10% of the road will

accommodate the task

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Cross section elements (continued)

Shoulder width There is some evidence that crash rates

reduce as shoulder width increases• An American study 1981showed a 21% reduction

in total crashes when a road with no shoulders had shoulders of 3-9 feet

• The study suggest that for the roads currently without shoulders, the optimum shoulder width to be provided is 5 feet

• A Swedish study showed decrease in crashes with increase shoulder width from 0 to 7 ft, and a little additional benefit was obtained for shoulder width above 8 ft

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Cross section elements (continued)

Lane and shoulder width The effects of lane and shoulder width are

not independent Widening lanes from 9 to 12 ft without

shoulder improvements reduce crashes by 32%

The greatest gain comes from the combination of lane and shoulder improvements• Widening a highway from 9 to 12 ft lanes and 0

to 6 ft shoulders, reduced crashes by about 60%

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Cross section elements (continued)

Surface cross slope Drainage is an essential part of any road 6 mm film of water can reduce friction

coefficient to near zero, thus making braking and turning almost impossible

Most wet weather crashes occur on low skid resistance pavements

Dunlap 1978 found that thickness of a water film on large radius curves can be almost twice that on a crowned straight section of road with the same cross slope

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Sight distance A Swedish study 1990 found that in most

cases crash rate decreases with increasing average sight distance especially single vehicle crashes at night

McBean 1982 found that on rural roads in Britain, sight distances shorter than 700 ft were more likely to be found at crash through their association with horizontal curves

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Sight distance (continued) A study reported by TRB, went on to develop a

model to assist in determining the cost-effectiveness of lengthening a vertical curve to increase sight distance over a crest It is cost-effective when the design speed is

more than 20 mph below operating speed in the area

Traffic flows exceed 1500 vpd High volume intersection Sharp curve Steep downgrade Lane drop

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Sight distance (continued) Improving sight distance on horizontal

curves has been found to be highly cost-effective If involves low cost treatments

• Clearing vegetation• Clearing Minor obstructions

Significant truck volume present• Since larger and heavier trucks have poorer

braking performance despite increased eye height, it must be compensated by greater sight distance

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Horizontal and vertical alignment

Average crash rate for curved road segment is three times that of straight segment

Average run-off-road crash rates is four times higher in curved segment than tangent

Studies suggested that horizontal curves on rural roads should not be less than about 2000 ft radius

A significantly higher crash rate can be expected on curves with radius below 1500 feet

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Horizontal and vertical alignment (continued)

From safety viewpoint, the important issue is consideration of this factor in a consistent fashion with other design parameter along the stretch of road

Curve flattening is expensive and is only cost-effective under certain condition

Other treatments for safety problems at horizontal curve include Physical rehabilitation and partial reconstruction

• Removal of roadside hazards• Trees• Utility poles

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Horizontal and vertical alignment (continued)

Flattening the side slope Resurfacing the roadway to improve skid

resistance Increasing the superelevation Paving the shoulders Eliminating pavement age drops

Low cost treatments will include Upgrading the pavement edge lines and

centerlines Adding raised reflective pavement markers Providing curve alignment markers

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Horizontal and vertical alignment (continued)

Upgrading the advance warning Grades should ideally not exceed 6%, with a

lower value 4% when there is a high proportion of trucks using the road

The worst situation happens when some road feature such as sharp curves, steep grades and intersections come together or in close proximity to each other

Horizontal curves should utilize plan transitions to connect the straight with circular arc, particularly on the roads with high proportion of trucks

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Bridges, structures and culverts

Bridges and culverts can be significant in terms of in their involvement in run-off-road crashes

For new bridges, Mak 1987 has recommended that the bridges should be 6 ft wider than traveled way, 3 ft

shoulders should carried across the bridge On roads with high traffic flows, full width

shoulders may be carried across the bridge Overpasses should have bridge piers which are

designed for impact loading, ideally there should be no pier at the edge of the roadway

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Overtaking If there is guard fencing alongside the road on

the bridge approaches, this will design to yield on impact

• There is a need for transition in stiffness of the guard fencing adjacent to the bridge end post, and the guard fence should be rigidly attached to the end post

Overtaking associated with crashes on rural two-lane roads

Overtaking lanes provide significant safety benefits

In Australia, the recommendations for a two-lane rural road with design speed of 60 mph is to have

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Truck escape ramps A minimum length of overtaking lane of about

2000 ft and maximum of 4000 ft The total taper length of 800 ft

Installation of an escape ramp is one of the few safety treatments designed for trucks to reduce the hazard of a runaway truck on a downgrade

There are six different types of design of truck escape ramp Sand pile Gravity ramp Ascending-grade arrester bed

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Truck escape ramps (continued)

Descending-grade arresting bed Horizontal-grade arrester bed Roadside arrester bed

FHWA quote a study in Colorado involving before and after study of the effectiveness of construction of truck escape ramps The most successful ramp showed a 400% reduction

in crashes and benefit-cost ratio of 10:1