Introduction to Highway Design

Introduction to Highway Design

• Unit 3 discusses the introduction to highway design, hierarchy of layouts, design speed, sight distances, planning, costings, and environmental aspects.

What This Unit Covers

• Typical highway cross-sections and road hierarchy are covered
• Underlying considerations for highway design are conveyed
• The DMRB tables are used to calculate design speed for a highway

Highway Components

• Verge
• Hard shoulder
• Main Carriageway
• Hard strip
• Central reservation

DMRB (Design Manual for Roads and Bridges)

• Defines requirements for all aspects of road infrastructure
• Includes geometric design/road layout
• Pavement design
• Structures
• Drainage
• Geotechnical aspects
• Freely available online

Road Hierarchy and Carriageway Width

• S2: 2-lane single carriageway is 7.3 m wide
• WS2: 2-lane wide single carriageway is 10.0 m wide
• D2AP: 2-lane all-purpose dual carriageway is 7.3 m wide
• D3AP: 3-lane all-purpose dual carriageway is 11.0 m wide
• D2M: 2-lane motorway is 7.3 m wide
• D3M: 3-lane motorway is 11.0 m wide
• Any reduction or increase should only occur in exceptional instances like cyclist accommodation or unavailable land

Road Costs

• Prices include: earthworks, structures, drainage, pavement, line markings, reflective studs, signage, lighting, motorway communications, fencing and landscaping
• Prices do not include: side roads, interchanges, underbridges, overbridges, culverts, subways, retaining walls and land cost
• Spon's Civil Engineering and Highways Works pricing guide indicates costs for general, civil engineering, and highway work
• A full breakdown of labor, plant, and material elements is offered, with updated rates

Design Speed for Highways

• Highways must be safe and efficient
• Route design accounts for efficiency, minimized disruption, predicted traffic, and horizontal/vertical alignment based on design speed

Design Speed Concept

• Central to the design process
• Serves as a guide for selection of physical features
• Correct selection ensures safety and economy
• Chosen design speed must be consistent with anticipated vehicle speeds, normally:
  • 50km/hr
  • 60km/hr
  • 70km/hr
  • 85km/hr
  • 100km/hr
  • 120km/hr
• Acceptable if 85% of drivers travel at or below the designated design speed, inducing a situation where ~99% of drivers travel at or below one speed category above

Design Speed Percentiles

• The 50th percentile describes a speed at which 50% of vehicles do not exceed, and the 85th percentile dictates 85% of vehicles will not exceed
• Research suggests the safest group are those travelling at/below the 85th-90th percentiles
• Research indicates crash risk alters with speed
• Drivers falling between the 85th-90th percentile often show above-average skill, with lowest crash risk
• Crash risk significantly increases below the 30th percentile, speeds often attributed to less skilled drivers

Design Speed Factors

• The 85th percentile speed concept theory suggests the majority of drivers are:
  • Reasonable and prudent
  • Do not want to crash
  • Desire reaching destination in the shortest time
• A speed at/below which 85% of people drive under good conditions is considered the maximum safe speed

Sight Distances

• It is a function of the carriageway length a driver can see in horizontal and vertical planes
• Includes
  • Stopping distance
  • Overtaking distance
• Stopping Sight Distance = Perception distance + Reaction distance + Braking distance

Perception-Reaction Time

• The length of highway travelled during perception-reaction time is calculated by Perception-reaction distance (m) = 0.278tV
  • V = initial speed (km/hr)
  • t = combined perception and reaction time (s)
• General rate of deceleration is 0.25 g for highway design in the UK, achievable on normally-textured surfaces in wet conditions without discomforting occupants
• Braking distance (m) = v²/2w
  • v = initial speed (m/s)
  • w = rate of deceleration (m/s²)

Full Overtaking Sight Distance

• Applies to single carriageways
• There is no FOSD for highways above 120 km/hr as this is unsuitable for single carriageways

Design Speed Selection (urban)

• In the UK, the design speed on an urban highway is chosen based on speed limit, allowing for an exceedance margin
• Posted speed limits of 48, 64, 80 or 96 km/hr correspond with design speeds of 60B, 70A, 85A and 100A respectively

Design Speed Selection (rural)

• Three factors must be considered:
  • Mandatory Speed Constraint
  • Layout Constraint (L)
  • Alignment Constraint (A)

Layout Constraint (Lc)

• The constraint depends on road cross-section, verge width, and junction/access point frequency
• Low: 0-5 accesses per km
• Medium: 6-8 accesses per km
• High: > 9 accesses per km

Alignment Constraint (Ac)

• The Alignment Constraint (Ac) results from highway alignment as follows:
  • Dual Carriageways: Ac = 6.6 + B/10
  • Single Carriageways: Ac = 12 – VISI/60 + 2B/45
    • B = Bendiness in degrees/km
  • VISI = Harmonic mean visibility–can be estimated from: log10VISI = 2.46 + VW/25 – B/400
    • VW = average verge width

Basic Elements of Scheme Assessment

• Functional effectiveness of a highway is measured by the ability to assist and accommodate vehicle flow with both safety and efficiency
• Some properties to assess:
  • Quantity of traffic.
  • Type of vehicles within the traffic stream
  • Distribution of flow over a period of time.
  • Average speed of the traffic stream
  • Traffic flow density
• Parameters influence scale/layout, along with construction material type/quantity

Process for Carriageway Selection in the UK

• The process is summarized as:
  • Estimate 24-hour annual average daily traffic (AADT) for the opening year, accounting for induced traffic
  • Compare flow quantities with specified carriageway standards
  • Choose carriageway standards within value ranges above
  • Consider such local factors as construction cost, maintenance and network effects.
  • Conduct economic/environmental assessments to select optimal standard

AADT Value Calculation

• Divide traffic into five categories:
  • Cars
  • Light goods vehicles (LGVs):
  • Other goods vehicles(OGV 1)
  • Other goods vehicles (OGV 2)
  • Buses and coaches (PSV)
• Each category has separate growth rates for future years

Cost-Benefit Analysis

• Economic appraisal of highway schemes should be carried out
• Developer will require economic assessments of route options compared to performance against the "do-nothing" alternative, and/or the "do-minimum" alternative
• Extensive redesign would be termed "do something"

Cost-Benefit Analysis Steps

• Steps include:
  • Identifying the project's options
  • Identifying relevant costs and benefits
    • Reductions in vehicle operating costs because new/upgraded project leads to lower congestion/higher speeds than existing roadway
    • Savings in time
    • Reduction in frequency of accidents.

Environmental Appraisal

• Environmental Evaluation requires an alternative analysis because cost-benefit analysis does not address effects on the environment
• A structure developed is known as the Environmental Impact Assessment (EIA)
• The Design Manual for Roads and Bridges provides a format for assessing highway schemes

Environmental Parameters

• There are 10 that must be assessed:

  • Air Quality (LA 105): Projects can significantly affect human health/designated habitats

  • Cultural heritage (LA 106): The demolition/disturbance of archaeological remains

  • Landscape and visual effects (LA 107): The iterative planning analysis must aim to minimize such effects

  • Biodiversity (LA 108): The proposed highway’s impact to flora and fauna

  • Geology and Soils (LA 109): such as effects on bedrock geology, superficial deposits, effects on soil resources

  • Material Assets and Waste (LA 110): associated with consumption/use of material assets