Elements of a Typical Cross-section of

Road and Highway drainage

Transportation Engineering – IDr. Attaullah Shah

Road

• A way or path over which cyclists, vehicles and pedestrians can pass lawfully.

• Roads are normally used for transportation within a country.

Advantages of Roads

• Nearest to the man, as for going to airport, harbor or railway station.

• Can be used by all types of vehicles from cycles to trailers.

• Can lead to any remote area and road users have freedom of movement.

• Vehicle movements are not time bound, roads are open to traffic for 24 hours.

Typical Road Cross-Section

Typical Road Cross-Section

Cross-Section Elements

The cross section of a road includes some or all of the following elements:

• Traveled way • Roadway • Median • Shoulder• Kerb• Traffic Barriers• Bicycle and pedestrian facilities • Drainage channels and side slopes

Two Lane Rural Highway Cross-Section

Urban Highway Cross-Section

Selection of appropriate cross-section elements

In selecting the appropriate cross-section elements and dimensions, designers need to consider a number of factors:

• Volume and composition (percent trucks, buses, and recreational vehicles) of the vehicular traffic expected to use the facility

• The likelihood that cyclists and pedestrians will use the route

• Climatic conditions

Selection of appropriate cross-section elements

• The presence of natural or human made obstructions adjacent to the roadway (e.g., rock cliffs, large trees, wetlands, buildings, power lines)

• Type and intensity of development along the section of the highway facility that is being designed

• Safety of the users The most appropriate design is the one that balances the mobility needs of the people using the facility (motorists, pedestrians, or cyclists) with the physical constraints of the corridor within which the facility is located.

Right of Way

• The right of way can be described generally as the publicly owned area of land that encompasses all the various cross-section elements.

• The right of way is the land set aside for use as a highway corridor.

• Rights of way are purchased prior to the construction of a new road, and usually enough extra land is purchased.

• Sometimes, rights of way are left vacant after the initial roadway facility is constructed to allow for future highway expansion.

Right of Way

Requirements of area for right of way are as follows:

• For 2 lane road = 150 ft width of area

• For 4 lane road = 250 ft width of area

• For 8 lane road = 300 ft width of area

Traveled Way or Carriage Way

• The portion of the roadway provided for the movement of vehicles, exclusive of shoulders.

• Number of lanes on a traveled way are decided on the basis of expected traffic volumes and appropriate level of service required for the facility.

Traveled Way or Carriage Way

• Lane width strongly influences traffic safety and comfort

• Lane width ranges from 2.7-3.6 m with 3.6 m lane predominant on high-type highways

• Two-lane two-way highways with the 3.6 m lane provide safe clearance between large commercial vehicles

Dual Carriage Way

When traffic volumes are quite heavy, carriageway may be divided into two parts by providing a median strip and each portion of the carriage way is reserved for traffic moving in opposite direction.

Road Way

• The portion of a highway provided for vehicular use.

• It includes both carriageway and shoulders.

Formation Width

• It is the sum of widths of carriage way, shoulders and median strips if provided.

• In case of embankments, it is measured as the top width.

• In case of cutting, it is the bottom width of the cutting from which side drains are excluded.

Shoulders

• Shoulders are the strips provided on both sides of the carriage way.

Functions of shoulders

• accommodation of stopped vehicles (disabled vehicles, bus stops)

• emergency use• lateral support for the pavement• space for roadside facilities• space for bicycles and pedestrians• driving comfort (freedom from strain)• improvement in sight distance• improvement in capacity

Width of shoulders

• Low-type roads -- minimum 0.6 m, recommended 1.8-2.4 m

• Shoulder provided for bicycles -- minimum 1.2 m wide

• High-type roads -- minimum 3.0 m, recommended 3.6 m

• Shoulders should be continuous.• Shoulders on bridges should have the

same width as on the approach sections.

Median or Traffic Separators

• It is the physical or painted separation provided on divided highways between two adjacent roadways.

• Medians can also be used to isolate slow and fast moving traffic in the same direction.

• Width of medians ranges from

1.2 to 24 m.

Median

Function of medians

• separate opposing traffic• recovery area for out-of-control

vehicles• stopping area• storage of right-turning and U-turning

vehicles• minimize headlight glare• provision for future lanes

Kerb

• It is the dividing line between carriageway and footpath.

Functions of Kerbs

• drainage control

• roadway edge delineation

• right-of-way reduction

• delineation of pedestrian walkways

• reduction in maintenance operation

• Assistance in roadside development

Types of Kerbs

• Class I Kerb: Height 7-9 cms

• Class II Kerb: Height 15-20 cms

• Barrier: 23-45 cms

• Submerged: provide lateral stability

Foot Path or side Walk

• Foot paths are provided in Urban roads

• These are raised strips constructed along both the edges of roads.

• Their minimum recommended width is 1.3 m.

Foot Path or side Walk

Foot Path or side Walk

Bicycle and Parking lane

• Bicycle lane is a portion of the roadway designated by striping, signing, and/or pavement markings for preferential or exclusive use by bicycles and/or other non-motorized vehicles.

• Parking lane is an additional lane provided on Urban roads and streets for on-street parking.

Bicycle and Parking lane

Minimum Width Requirements

Bicycle and Parking lane

Traffic Barriers

A longitudinal barrier, including bridge rail, or an impact attenuator used to

• Redirect vehicles from hazards located within an established Design Clear Zone

• To prevent median crossovers• To prevent errant vehicles from going over

the side of a bridge structure • To protect workers (occasionally) • To protect pedestrians, or bicyclists from

vehicular traffic

Traffic Barriers

Traffic Barriers

Drainage Channels and Side slopes

• Drainage channels and side slopes are provided along the length of road for storm water drainage etc.

Drainage Channels and Side slopes

Drainage channels should:• have adequate capacity for the design

runoff,• minimize damage to the highway caused

by unusual storm water,• minimize risk for motorists,• be resistant to the high speed water flows

where expected,• prevent sedimentation of the particles

carried by water.

Drainage Channels and Side slopes

Side slopes should• insure the stability of the roadway• provide opportunity for recovery of an out-

of-control vehicle

Part 2 Highway Drainage

Transportation Engineering - I

Highway Drainage

• A means by which surface water is removed from pavement and ROW

• Redirects water into appropriately designed channels

• Eventually discharges into natural water systems

Inadequate Drainage

• Damage to highway structures

• Loss of capacity

• Visibility problems with spray and retro-reflectivity

• Safety problems, reduced friction and hydroplaning

Highway Drainage

• Transverse slopes– Removes water from pavement surface– Facilitated by cross-section elements

(cross-slope, shoulder slope)

• Longitudinal slopes– Minimum gradient to maintain adequate

slope in longitudinal channels

• Longitudinal channels– Ditches along side of road to collect

surface water after run-off

Transverse slope

Longitudinal slope

Longitudinal channel

Drainage System

Three phases1. To Estimate the quantity of water to

reach the system

2. Hydraulic design of system elements

3. Comparison of different materials to serve the purpose

Steep slopes provide good hydraulic capacity and lower ROW costs, but reduces safety and increases erosion and maintenance costs

Hydrologic Analysis

Q = CIA (english) or Q = 0.0028CIA (metric)

Q = runoff (ft3/sec) or (m3/sec)

C = coefficient representing ratio or runoff to rainfall

I = intensity of rainfall (in/hour or mm/hour)

A = drainage area (acres or hectares)

Undivided traveled ways (two- and multilane) on tangents and flat curves have a crown in the middle and slope downward toward both edges (camber). The downward cross slope may be a plane or rounded section (parabolic), or a combination of the two.

One-way traveled ways on divided highways may be crowned separately or may have a unidirectional cross slope/cross fall.

Transverse Slope

Transverse Slope

Transverse Slope

Transverse Slope

Crowns vs. Unidirectional Slopes

Type of Roadway

Pros Cons

Crowned separately

•rapid drainage during rainstorms

•difference between low and high points is minimal

•inlets and underground drainage (drainage towards the median)

•difficult design of at-grade intersection elevation

•use of such sections should be limited to regions with high rainfall

Unidirectional cross slopes

•more comfortable for drivers changing lanes

•drainage away from the median saves inlets and drains

•simplifies treatment of intersections

•drainage is slower

•difference between low and high points of the cross section is larger

Contradictory design controls

A steep lateral slope reduces water ponding and the width of the water flow along the curb.

A flat lateral slope reduces vehicles' drift towards the low edge.

Recommended design controls

Lateral drift of vehicles at high speed is barely perceptible on cross slopes up to 2%. The slope of 1.5-2.0% is acceptable on high-speed highways. In the areas of intense rainfall a maximum cross slope is 2.5%.

Crown section

Change in the cross slope of 3-4% causes swaying of high body vehicles. Rounded crowns reduce discomfort.

Cross Slopes on Tangents

Curbed Highways

The minimum slop values of 1.5-2 % in areas with intense rainfalls will cause wide sheet of water on the curbed traveled way.

Possible improvements:• parabolic cross section with increasing cross slope

towards the outer edges,• gutter along the curb with the cross slope larger

than on the traveled way,• on multilane traveled way, cross slope broken

along traffic lane edges, increasing from the minimum value on the innermost lane up to the maximum value on the outermost lane. This solution is used on uncurbed sections as well.

Drainage Channels and Sideslopes

Design considerations of highway drainage includes

• safety • good appearance • control of pollutants • economy in maintenance

This can be achieved by applying• flat side slopes• wide drainage channels• rounding

Drainage Channels Types of Drainage Channels

814

812

810808

806

804

802

800

794

798

800

802

804 806

808

Roadside channel

Intercepting channel

FlumeToe-of-slope channel

Drainage Channels

Drainage channels should:• have adequate capacity for the design

runoff,• minimize damage to the highway caused

by unusual storm water,• minimize risk for motorists,• be resistant to the high speed water flows

where expected,• prevent sedimentation of the particles

carried by water.

Side slopes

Side slopes should:• insure the stability of the roadway• provide opportunity for recovery of an out-of-control vehicles

Roadside Channels

• Steep sides improve hydraulic efficiency and reduce right of way costs

• Flatter sides improve slope stability and traffic safety, reduce maintenance costs

• Side slopes 1:4 or flatter provides a good chance of recovery for errant vehicles and relax drivers' tension (roadside channel is visible to drivers)

• Side slopes of 1:5 or 1:6 are recommended in the flat areas • Intercepting channels have a flat cross section form by a dike

made with borrow material• Median drainage channels are shallow depressed areas with

inlets• Flumes are open channels or pipes used to connect

intercepting channels or shoulder curbs with roadside channels• Channel lining prevents channels erosion caused by fast

stream of water.

Examples: grass (where possible), concrete, stone etc.

Side slopes

Safety consideration

• Rounded hinge point reduces the chance of an errant vehicle becoming airborne

• Fore slopes 1:6 or flatter can be negotiated by errant vehicles

• Fore slopes 1:3 with liberal rounding provide a good chance for recovery

• Slopes steeper than 1:3 can be used only where justified by local conditions. The use of roadside barriers should be considered

Maintenance consideration

• Flat and well-rounded side slopes simplify establishment of turf and its maintenance

• Slopes 1:3 or flatter enable the use of motorized equipment

Side slopes

Other rules

Flat, well-rounded side slopes create a streamlined cross section. Advantages for the streamlined cross sections are:

• natural, pleasant appearance,

• improved traffic safety,

• snow drift prevented,

• easy maintenance.

Retaining walls should be considered where slopes would be steeper than 1:2.

Standard slope for rock cuts is 2:1. In good-quality rock, slopes ranges from 6:1.

Sideslopes