Maintenance of Drainage 

Features for Safety

A Guide for Local Street and Highway 

Maintenance Personnel

July 2009

FHWA-SA-09-024
Notice

This document is disseminated under the sponsorship 

of the Department of Transportation in the interest of 

information exchange. The United States Government 

assumes no liability for its content or use thereof.

The contents of this report reflect the views of the 

author, who is responsible for the facts and the 

accuracy of the data presented herein. The contents 

do not necessarily reflect the official policy of the 

Department of Transportation. 

This report does not constitute a manual, handbook, 

standard, specification, or regulation. 

The United States Government does not endorse 

products or manufacturers. Any trademarks or 

manufacturers’ names that appear herein do so only 

because they are considered essential to the objective 

of this document.
1. Report No. FHWA 

FHWA-SA-09-024

2. Government Accession 

No.

3. Recipient’s Catalog No.

 

4. Title and Subtitle

Maintenance of Drainage Features for Safety, A Guide for Local Street and 

Highway Maintenance Personnel

5. Report Date

July 2009

6. Performing Organization 

Code

 

7. Authors

Hugh W. McGee, P.E., Daniel Nabors, P.E., and Timothy Baughman, P.E. 

(editors).

8. Performing Organization 

Report No.

9. Performing Organization Name and Address 10. Work Unit No.

Vanasse Hangen Brustlin, Inc

8300 Boone Boulevard, Suite 700

Vienna, VA 22182-2626

11. Contract or Grant No.

DTFH61-05-D-00024

12. Sponsoring Agency Name and Address

Office of Safety

Federal Highway Administration

U.S. Department of Transportation

1200 New Jersey Avenue, S.E.

Washington, D.C. 20590

13. Type of Report and Period 

Covered

Final Report

14. Sponsoring Agency Code

15. Supplementary Notes: The FHWA Office of Safety Contract Task Order Manager was Ms. Karen Timpone. 

The document was originally prepared by staff at FHWA as a revision to the 1990 edition of the same title FHWART-90-005. Those identified as authors above provided additional material and served as technical editors..

16. Abstract

This guide was prepared to help local road agency maintenance workers understand the importance of maintaining 

and upgrading drainage features on their road system. Storm water that stands or ponds on the road surface and 

shoulders can contribute to crashes. It can cause hydroplaning under water conditions and skidding under icing 

conditions. Water run-off can deteriorate the shoulder, sideslopes, and reduce the effectiveness of safety hardware 

(guardrails, sign posts, etc.) This guide identifies typical drainage problems and suggests corrective measures to 

improve safety.

17. Key Words

Vegetation control, safety, maintenance, sight 

distance

18. Distribution Statement

No restrictions. This document is available to the public through 

the National Technical Information Service, Springfield, VA 

22161.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. 

No. of 

Pages: 

38

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized

i
TABLE OF CONTENTS

I. INTRODUCTION................................................. 1

II. HOW STORM RUN-OFF AFFECTS ROADWAY SAFETY .... 2

Travel Way Pavement Surface ............................................2

Ditches and Side Slopes....................................................6

Drainage Structures ........................................................6

Pedestrian and Bicycle Paths .............................................7

III. RECOGNIZING DRAINAGE PROBLEMS.....................10

IV. CORRECTING UNSAFE DRAINAGE FEATURES ...........12

Correcting the Effects of Erosion on Roadway Side Slopes ....... 12

Correcting the Effects of Erosion on Roadway Hardware......... 13

Correcting the Effects of Erosion on Culvert and Pipe Ends ..... 14

Correcting the Effects of Erosion on Ditches........................ 14

Maintaining and Improving the Safety Characteristics of Inlets . 17

Eliminating or Improving Hazardous Drainage Headwalls......... 19

Improving Hazardous Pipe and Culvert Ends ........................ 20

Maintaining Good Drainage at Intersections and Access Points .. 22

V. WORKER SAFETY .............................................24

VI. REFERENCES .................................................25

APPENDIX A: CLEAR ZONE DESCRIPTION....................26

APPENDIX B. THREE TYPICAL WORK ZONE LAYOUTS 

FROM MUTCD.....................................................28

ii
1

I. INTRODUCTION

Safety on streets and highways, bicycle trails, and sidewalks 

is essential to motorists, bicyclists, and pedestrians. Drainage 

systems that remove storm water run-off from streets and 

highways are an integral feature of a safe system. Water that 

remains on the roadway1 surface can contribute to vehicle 

hydroplaning. In winter, standing water can freeze and cause 

skidding.

The curbs, gutters, channels, and ditches that carry the run-off 

away from the roadway can have a serious effect on an errant 

motorist or bicyclist when not designed and maintained correctly. 

Erosion along the roadway can also contribute to the severity of 

a crash or inundate crashworthy systems such as breakaway sign 

supports. Even headwalls, pipe ends and grates on drop inlets and 

pipe openings need to be safety treated when they are within the 

area an errant motor vehicle or bicycle can reach.

Maintaining roadway drainage is important for safety and for 

ensuring the long life of the roadway by:

h Preventing erosion of the roadway.

h Preventing saturation of the subbase.

h Preventing damage to roadway structures.

This guide is intended to help local road agency maintenance 

workers understand the importance of maintaining and 

upgrading drainage features on their road system to avoid an 

unsafe condition. Its purpose is to highlight common roadway 

drainage problems that can cause an unsafe condition and suggest 

inspection methods and corrective action. This guide is not 

intended to be a design guide. Readers may want to contact their 

State Local Technical Assistance Program (LTAP) for more details 

on drainage design.

1 In the context of this guide, the roadway includes the travel lanes and shoulders, 

if any. Streets and highways are considered roadways or roads. The travel way is that 

portion of the roadway for vehicle travel.
2

II. HOW STORM RUN-OFF AFFECTS 

ROADWAY SAFETY

There are many factors affecting safety on local streets and 

highways, and one of these is how drainage—the run-off of water 

from the pavement and shoulder—is treated.

The primary objective in treating drainage is to ensure that 

safe operating conditions exist on the roadway. This is done 

by removing storm run-off, providing for snow removal and 

reducing the ability for ice to form on roadways and bridges. 

Additionally, the drainage systems used on the roadway should 

not be hazardous or reduce the crashworthiness of other systems, 

such as breakaway signs and guardrail.

Also included as part of this objective is the safe design and 

maintenance of drainage features, such as travel way and shoulder 

surfaces, side slopes, drop inlets, pipe ends, ditches, headwalls, 

safety grates, culverts, gutters, roadside and median barriers and 

other features that restrict the flow of water or removal of snow. 

Travel Way Pavement Surface

Drainage factors that affect safety on the travel way of streets and 

highways include:

h Even a thin layer of water on the travel way surface can 

initiate motor vehicle hydroplaning at speeds as low as 35 

MPH. When a vehicle tire rides on top of this thin layer of 

water, the vehicle cannot be steered or stopped easily. A 

vehicle that is hydroplaning behaves similarly to a vehicle 

on ice.

h Water ponding2 in the wheel paths or ruts is also very 

dangerous, particularly for motorcycles and bicycles. 

Water ponding at intersections may be especially 

hazardous as vehicles may require greater distances to 

stop.

2 Ponding refers to water collecting on the pavement or shoulder that is deteriorated. 

Standing water refers to water collecting on the surface due to insufficient cross slope or 

backup in the drainage system.
3

Travel ways are generally designed to provide sufficient cross 

slope to facilitate storm run-off; however both traffic and 

maintenance activities can and often do affect the original cross 

slope of the roadway surface. Traffic, particularly heavy truck 

traffic, can cause wheel ruts in the pavement surface.

Water ponding in wheel ruts.

Rutting is caused by the wear or movement of the bituminous 

pavement to the sides of the wheel path. Rutting usually runs 

parallel to the centerline of the roadway. When wheel ruts are 

noticeable, they are generally deep enough to pond sufficient 

water to initiate hydroplaning. 

Another pavement condition, often referred to as shoving, can 

occur where vehicles frequently stop, such as at intersections. 

Shoving is most common at stop locations, particularly on steep 

downgrades. Shoving results in a series of ridges running across 

the roadway. The ridges can retain water and are hazardous even 

when dry because they cause the wheels of a vehicle to bounce, 

reducing wheel contact with the roadway surface and decreasing 

braking ability. 

Ponding or standing water in the travel way may also cause some 

drivers, bicyclists, or pedestrians to divert from their desired path. 

Storm drains or drop inlets that have collected debris, such as 

leaves, can cause ponding during and after storms. Both motor 

vehicle and bicycle operators may choose to avoid going through 
4

the water by encroaching on the opposing lane, thus endangering 

themselves and opposing traffic. 

Water standing in the roadway.

In sections of the travel way where run-off drains directly onto 

the shoulders (where there are no curb and/or gutters), water may 

collect along the edge of the travel way. Water on a portion of 

roadway can result in drivers losing control of their motor vehicle, 

particularly when braking in an emergency. This can happen 

when the inside tires are in contact with roadway surface while 

the braking ability of the outside tires is hindered by the water. 

Water can pond on the outside 

edge of the travel way surface 

when debris, particularly 

aggregate and soil on turf 

shoulders, builds up. As debris 

accumulates on the shoulder, it 

raises the level of the edge, and 

eventually hinders run-off from 

flowing into side ditches.

Water ponding at the edge of 

roadway.
5

Water ponding on the edge 

of the pavement contributes 

to the deterioration of the 

pavement edge and the rutting 

of stabilized soil supporting 

the pavement edge, which 

can result in additional safety 

hazards. Edge drop-offs and 

shoulder scour are often caused 

when water is trapped at the 

pavement edge by the buildup of debris and vegetation 

growth. 

Partial overlays and pavement repairs can result in water being 

trapped and retained on the travel way surface. Partial overlays, 

either to correct shoulder deterioration or widen the roadway 

surface, result in a pavement edge where the overlay stops. 

Depending on the size of aggregate in the overlay mix and the 

effort taken to feather the lip into the existing pavement, water 

can be retained on the travel way. When the lip is along the wheel 

path, the thin layer of retained water can initiate hydroplaning, 

reduce braking ability or freeze and contribute to skidding.

The edge of this partial pavement overlay is 

causing water to be retained on the travel 

way surface.

Deterioration of 

pavement edge and 

shoulder due to poor 

drainage.
6

Ditches and Side Slopes

The area along the side of the roadway is also affected by run-off. 

Ditches and side slopes (either foreslope or backslope) are affected 

by storm run-off and maintenance activities. Motor vehicles and 

bicycles occasionally run off the roadway, but when the side 

slopes are relatively flat (1 vertical:10 horizontal [1V:10H] slope 

or flatter) and well graded (free of erosion scars) and ditches 

maintained with traversable side slopes, most drivers can recover 

control of the vehicle and return to the roadway or come to a safe 

stop. 

One of the reasons that run-off-the-road errant vehicles may not 

be involved in a severe crash is safer shoulders and forgiving 

roadsides. Many roadways are designed to allow run-off to travel 

across the shoulders and be collected in a ditch or channel or flow 

down the side slope and seek a natural channel. 

Drainage ditches are common on roadways when curbs are not 

used to channelize storm run-off. There are also cut-off ditches 

used along the travel way to prevent run-off from adjacent land 

flowing onto the travel way.

Drainage ditches should be designed so a vehicle leaving the 

roadway can cross over them without the vehicle overturning, 

being abruptly stopped or causing the driver to lose control. 

Drainage ditches over which a motor vehicle can safely drive are 

called traversable. Design criteria for traversable drainage ditches 

are provided in the current edition of the Roadside Design Guide

(see references at the end of this guide). 

Ditches need to be cleaned on a regular basis to prevent them 

from silting up and forcing water back onto the travel way surface 

or into the subbase of the pavement.
7

Earth ditches are subject to erosion. The ruts and deposits of 

silt and debris can change the shape of the ditch, resulting in 

sections that are no longer traversable or forgiving. Additionally, 

maintenance activities, such as cleaning ditches, can result in 

creating a non-traversable ditch.

Side slopes along the travel way may also become eroded, 

particularly when ditches silt up. The steeper side slopes (1V:3H 

and 1V:4H) that are marginally traversable become hazardous 

when they develop erosion channels that can trip or snag a tire.

A traversable drainage ditch 

such as this provides space 

for vehicle recovery.

A non-traversable drainage 

ditch such as this is a safety 

hazard.

This regraded earth shoulder and ditch has a steep 

backslope which makes it non-traversable; if right-of-way 

is available the backslope should be made less steep.
8

Drainage Structures

In addition to ditches and side slopes, incorrectly maintained drop 

inlets, pipe ends, culvert ends, head walls, and other drainage 

features located adjacent to the roadway may be potentially 

hazardous.

Pipe ends, culvert wing walls, and headwalls adjacent to the 

roadway are potentially hazardous when they extend above the 

surrounding ground. These features are rigid objects that can 

snag the undercarriage of a vehicle leaving the roadway or initiate 

vehicle rollover. 

Many of the older drainage features were built above the ground. 

These features are rigid objects that can cause serious injury if 

hit. Current practice provides for traversable ends, extension 

or relocation of the ends far enough away from the roadway to 

reduce the risk of a crash or use of a barrier system to shield road 

users from a hazardous location.

Even traversable drainage features can become hazards when 

storm run-off or maintenance operations, such as dressing side 

slopes, result in the feature extending above the surrounding 

ground. These features are rigid objects that can snag the 

undercarriage of a vehicle leaving the roadway or initiate vehicle 

rollover.

The headwall is sticking up 

almost a foot in a relatively 

flat recoverable area. It can 

snag a vehicle and bring it 

to an abrupt stop or cause it 

to overturn.

This headwall is a roadside 

obstacle and should be replaced 

as soon as possible.
9

Pedestrian and Bicycle Paths

The safety of pedestrians and bicyclists can also be jeopardized by 

poor drainage in the travel way, shoulder, or paths. While these 

users are more able to avoid or compensate for standing water or 

ice in their path, maintenance activities and relatively low-cost 

improvements can prevent drainage problems and improve the 

safety for these users. 

This water ponding on a ped/bike path 

could have been avoided by proper 

grading of the path.
10

III. RECOGNIZING DRAINAGE PROBLEMS

The drainage problems discussed in the previous section can 

directly cause or contribute to crashes. As an example, drainage 

features that fail to remove run-off because they are too small 

or are clogged and pond water on the roadway can cause 

hydroplaning or force drivers to leave their lane. Additionally, 

other drainage features which do not have anything to do with 

causing a crash can significantly contribute to the severity of the 

crash, such as an errant vehicle striking a culvert headwall.

It is important to identify these 

potentially hazardous situations 

as soon as possible. Some of 

these conditions may have been 

in existence for quite some time, 

while others may have recently 

developed as a result of a storm 

or change in weather conditions. 

For areas of the country that 

are subject to freezing, the 

spring thaw frequently reveals 

pavement, shoulder and roadside 

deterioration. 

Drainage problem locations can be 

identified in several ways: 

h Citizen complaints—Citizens drive the roads every day 

and are the first to notice problems. You should have an 

easily identified phone number, e.g. on your web site, 

where citizens can call in problems and complaints.

h Local police—Your local police patrols can also 

identify drainage problems. Police personnel need to 

be instructed on what types of drainage problems they 

should be on the ‘look out’.

h Crash data—A periodic review of crash data can reveal 

some locations where drainage deficiencies may be a 

contributing factor. Locations that experience a higher 

than average number of wet weather crashes may have 

deteriorated pavement surfaces. Also look for crashes 

A clogged drain can easily 

be detected during a field 

survey. 
11

with fixed objects, which could include drainage 

structures that need to be removed or protected by a 

barrier.

h Field reviews—Potentially hazardous drainage features 

are best identified through field reviews. Drainage 

features may be reviewed alone or in a combined safety 

review. An excellent time to review a road or section 

of highway is when there is other highway work being 

planned. For example, if a section of road is scheduled for 

an overlay, the section may be reviewed to determine if 

there are any safety improvements that can be included. 

Including drainage improvements on other projects will 

often reduce the cost of the work and the inconvenience to 

the public.

FIELD INSPECTION CHECK LIST

(CHECK FOR THE FOLLOWING CONDITIONS)

1. Rutting or shoving of pavement surface.

2. Discontinuity of surface level between the pavement 

and shoulder (shoulder drop-off).

3. Accumulation of earth or debris on shoulder.

4. Existence of erosion channels on ditch side slopes.

5. Silt or debris accumulation in ditch.

6. Headwalls and drainage structures that are not flush 

with the ground.

7. Damage to drainage structures, such as crushed 

culverts.

8. Grates with wide openings parallel to the roadway that 

can trap bicyclists.

9. Drains blocked by soil and debris.

10. Side slopes that have steepened due to erosion.

11. Erosion around all roadside structures such as 

headwalls, sign posts, and guardrail posts.

12. Drainage structures within clear zone that are not 

traversable or protected by suitable barrier system.
12

IV. CORRECTING UNSAFE DRAINAGE 

FEATURES

There are many drainage features on and along streets and 

highways. They include the curbs and gutters, drop inlets and 

catch basins, pipes, culverts, and ditches needed to collect and 

carry storm drainage away from the road. When these features are 

located on a roadway or are adjacent to it where a vehicle, bicycle 

or pedestrian has a chance of traveling over or into the feature, the 

drainage feature should be designed for safety. 

Drainage features that have been designed for safety, crash 

tested, and found to operate safely under the design conditions 

are usually referred to as crashworthy. Basically, crashworthy 

means a highway feature or appurtenance will not stop a vehicle 

abruptly, cause the driver to lose control or cause the vehicle to 

roll over.

This section describes corrective measures that can be taken to 

mitigate the effects of an identified drainage problem that affects 

safety.

Correcting the Effects of Erosion on Roadway Side Slopes

Side slopes, both cut and fill embankments, and ditch sides, are 

usually constructed in the ranges of 1V:3H to lV:6H (or 1V:8H 

in relatively flat areas). Steeper slopes pose greater safety risk 

to motor vehicles traveling across or down them. These steeper 

slopes are also more difficult to maintain.

Side slopes of 1V:4H or flatter are commonly accepted as safe side 

slopes. These slopes are traversable and recoverable. Drivers of 

errant vehicles can steer, brake, and recover from a run-off-theroad mistake. 

Side slopes between 1V:3H and 1V:4H offer marginal safety for 

vehicles. These slopes are called traversable, but non-recoverable 

and vehicles will travel beyond the bottom of the slope; therefore, 

a recovery area should be provided at the bottom of the 

embankment.
13

All slope surfaces should be smooth, free of fixed objects, and free 

of snagging features, such as headwalls. Vehicles traveling down 

slopes are difficult to control and may strike, roll over or drop 

into a feature such as a pipe end, which can cause a vehicle to halt 

abruptly, become unstable and roll over, or strike the back of the 

slope.

Erosion scars on a side slope can also initiate vehicle instability 

by tripping the vehicle’s wheels and initiating overturning. 

Therefore, eroded slopes should be graded and seeded. When 

rip rap is used to control and spread the flow of water, it should 

have shallow inverts and be placed flush with the existing ground. 

Additionally, the roadside conditions should be checked to 

determine why erosion is taking place and the problems resolved.

Side slope erosion can cause rollovers.

Correcting the Effects of Erosion on Roadway Hardware

Soil erosion can have a detrimental effect on safety appurtenances, 

such as guardrail, sign supports, and highway light supports. The 

deposit of several inches of eroded soil or the erosion of several 

inches can significantly reduce crashworthy characteristics.

Soil deposited around breakaway appurtenances can cause them 

to malfunction or simply not break away. Additionally, run-off 

can result in the erosion of soil around these safety features, 

reducing their operational characteristics, such as deflection on 

impact rather than breaking away. This can cause a vehicle to roll 

over or abruptly stop.
14

Eroded and silted areas around barrier posts, breakaway sign 

supports and highway light supports should be restored to the 

desirable ground level and seeded. If the area continues to erode, 

bituminous overlays may be considered.

Desirably, drainage channels should not end near a safety feature 

or drain adjacent to one where soil can erode around it.

Correcting the Effects of Erosion on Culvert and Pipe 

Ends

Pipes and culverts may have headwalls or special pipe end 

sections. Headwalls tend to channel water around their edges 

and can cause erosion gullies to develop. The storm flow from the 

pipe or culvert can also erode the sides of a paved channel or the 

bottom of a graded channel. If these water channels erode, they 

can create gullies on the side slopes that can trip the wheels of 

an errant vehicle or bicycle causing instability, loss of control or 

initiating a vehicle rollover. 

Pipe and culvert ends should be checked annually or after major 

storms. Debris that can divert water flow should be removed 

and eroded areas reestablished with soil/aggregate mixtures 

and reseeded. Additional measurers may need to be taken to 

reduce erosion, such as paving the channel in areas where erosion 

continues.

Correcting the Effects of Erosion on Ditches

Roadside ditches and channels are often cut into the roadside 

immediately adjacent to the roadway. There are designs for 

ditches that ensure their traversability, meaning a vehicle or 

bicycle can pass over the ditch or channel at the travel way speed 

without abruptly stopping, losing control or being rolled over. 

Recommendations for ditch cross sections (front slope, bottom, 

depth, and back slope) can be found in the current edition of the 

Roadside Design Guide. The choice of a traversable ditch section 

depends on the amount of run-off, the grade of the roadway, slope 

and soil conditions and speed of vehicles on the highway.
15

Both earth and lined ditches require maintenance to remove 

debris and prevent erosion that can create roadside hazards 

and/or reducing the effectiveness of the drainage system. It is 

important when repairing eroded earth ditches and shoulders to 

restore them to their original safety shapes. Ditch side slopes that 

are too steep and ditches that are too deep can initiate instability 

in a vehicle causing it to roll over or cause the vehicle to snag 

against the ditch back slope and abruptly stop or vault the vehicle 

into the air.

Deep ditches such as this are a safety hazard.

Earth roadside ditches that continue to erode and require 

regrading one or more times a year should be considered 

candidates for paving or some other form of lining that eliminates 

erosion and reduces the speed of the run-off. Rock and stone ditch 

linings should be smooth and not “bumpy” so the driver can 

retain control of the vehicle.
16

Roadside ditches, along with the other drainage features such as 

drop inlets, should be checked annually and after major storms to 

ensure they are not clogged with debris or eroded.

Roadside ditches that are not traversable by design, as a result 

of previous maintenance actions or because of erosion, should 

be redesigned to be consistent with those recommended in the 

current edition of the Roadside Design Guide (see discussion under 

section 3.2.4 Roadside Channels for preferred cross section).

This ditch was reshaped to provide a safe 

recovery area for errant vehicles.

Erosion control measures such 

as fiber mats shown may be 

necessary to prevent ditch 

erosion.

For ditches with heavy runoff, concrete lining may be 

necessary.
17

Maintaining and Improving the Safety Characteristics of 

Inlets

Drop inlets and catch basins in and adjacent to roadways are one 

of the most common drainage features especially in urban and 

suburban areas where curb and gutter design is used. Inlets are 

designed to carry surface run-off from the road and roadside away 

from the roadway. Inlets can be of varying design, including curb 

openings, grates, or a combination of these. Many local agencies 

use a standard design for most of their drop inlets on and adjacent 

to low-speed, low-volume roads. Inlets on higher level highways 

are often specially designed or selected to meet the conditions of 

greater but less frequent storms.

Drop inlets located in or adjacent to the path of motor vehicles, 

pedestrians and bicycles require grates that can accommodate 

run-off while preventing vehicles, bicycles and pedestrians 

from falling into the inlet. When bicycle traffic exists, grates 

should prevent the tires of a bicycle from slipping into and being 

caught in the grate. Therefore, bicycle safe grates should be used 

whenever bicycle traffic is expected. The photo below shows a 

preferred treatment for a drop inlet located on or near a bicycle 

travel way. With a flush bicycle-safe grate, a bicycle tire cannot get 

caught in the grate because of the cross pattern of supports which 

results in only small openings.

Bicycle safe grate with drop inlet.

Drop inlets should not be in the path of pedestrians. To avoid runoff across a pedestrian crosswalk it is desirable to locate the catch 

basin and inlet before the crosswalk as depicted on next page.
18

To avoid run-off across the pedestrian path, it is desirable to 

locate drop inlets before crosswalks.

Raised roadside drop inlets, sometimes referred to as table top 

inlets, provide a vertical opening in their sides for large volumes 

of run-off and debris removal. They are usually located in lowdepressed or sump areas. Raised inlets located in areas where an 

errant vehicle or bicycle can drive into or over them should be no 

higher than 4 inches above the surrounding ground. Drop inlets 

that extend more than 4 inches above the surrounding ground 

can snag the undercarriage of a vehicle in the same way a raised 

headwall can, causing it to abruptly stop, go out of control, or 

roll over. A safer drainage treatment is to use a grated inlet that is 

close to the ground as shown below.

V-shaped grate in median.
19

Eliminating or Improving Hazardous Drainage Headwalls

Headwalls are common features 

on many local roadways. The 

headwalls are often used to 

support the shoulder and 

maintain the roadway edge, 

prevent the end of the pipe from 

being crushed or broken when 

overridden, collect and disperse 

water flows and occasionally 

delineate the ditch or channel. 

Headwalls, by their nature, are 

generally rigid structures capable 

of abruptly stopping a motor 

vehicle if hit. When they are 

located on side slopes they may 

be potential hazards, causing loss 

of control, ramping or vehicle roll 

over.

In flat areas, 1V:10H or milder, 

fixed objects such as headwalls, 

should not extend more than 4 

inches above the surrounding 

ground. Fixed objects that are 

higher than 4 inches can catch 

or snag part of a small vehicle 

undercarriage causing abrupt 

stopping, loss of control, and/or 

vehicle overturning.

Headwalls that are hazardous are often easy to identify by 

scars left on them from previous crashes. Headwalls that have 

been hit should be replaced with a traversable end treatment or 

crashworthy safety feature rather than replaced or repaired in 

kind. As an immediate, temporary treatment, the headwall should 

be delineated with a retroreflective object marker consistent with 

the current edition of the Manual on Uniform Traffic Control Devices.

This culvert headwall is a 

roadside hazard.

In flat areas the height of 

the headwall should not 

exceed 4 inches.
20

Improving Hazardous Pipe and Culvert Ends

Pipe and culvert ends vary widely. Many are just extensions of the 

pipe beyond an embankment. These ends are used because they 

are inexpensive and tend to limit the erosion around the sides 

of the pipe, or it may be that it was simpler to just leave the last 

section of pipe, rather than trying to dress it. Unfortunately, many 

pipe and culvert ends are within the area required by a driver to 

recover control or stop a vehicle that has left the travel way. 

When pipe or culvert ends are within this recovery area or “clear 

zone3,” it should be designed and maintained as a traversable 

feature of the roadside.

Several features of a pipe or culvert end can be hazardous to 

motorists, bicyclists or pedestrians that leave the travel way. These 

features include: (1) how well or poorly the pipe or culvert end 

conforms to the ground, (2) the size and shape of the end opening, 

and (3) the orientation and location of the pipe opening.

One of the best ways to ensure that the pipe or culvert end is not 

hazardous is to place the end substantially beyond the clear zone. 

Unfortunately, cost, geography, and/or right-of-way restrictions 

can be factors that limit the extension of pipe and culvert ends.

3 See appendix for definition of clear zone.

Rigid headwall delineated with 

Type 3 Object Marker.
21

In areas where potential roadside hazards already exist, such as 

trees that are close to the travel way, pipe and culvert ends can be 

extended to beyond the tree line rather than in front of it. While 

this action does not reduce the existing hazardous nature of the 

roadside, it does not create additional hazards.

End sections within the clear zone of the roadside should be 

designed and maintained to be traversable features. First they 

should conform to the ground around them. An exposed end 

(pipe or culvert sections sticking up above the ground) can 

initiate vehicle instability and cause the driver to lose control. End 

sections more than 4 inches above the surrounding ground can 

snag the undercarriage of a vehicle, causing it to stop abruptly or 

to vault or roll over.

To minimize the potential of vehicle snagging on the end of a 

pipe, it is necessary to provide a traversable end section, ensuring 

that either the pipe end is cut flush with the ground line, or when 

used, that the steel grating is sloped flush with the surface. 

Grated inlet flush with ground over a culvert end.

A second safety consideration, important to all large pipe and 

culvert ends located in the clear zone, is the size or width of 

the opening. Pipes and culverts can trap a vehicle wheel if 

the opening is too large, causing it to stop abruptly or initiate 

overturning. Generally, openings that are larger than 30 inches 

are considered potential hazards; however vehicles may safely 

traverse over openings up to 30 inches. In crash tests of vehicles 

at normal highway speeds it was demonstrated that vehicle tires 
22

could cross over openings of 30 inches provided that the opening 

has been cut to match a traversable side slope.

This culvert has a rather large opening which could 

cause a vehicle that hits it to go out of control.

Large openings of pipe and culvert ends can often be made 

traversable for a vehicle by construction of an open pipe grate 

across the potential path of the vehicle. Pipe grates need to be 

of sufficient strength to accommodate a vehicle and should be 

removable or hinged to provide for maintenance. Design of the 

pipe grate is important to safety and varies with the size, shape, 

location, and orientation of the pipe.

A potential source of information on safety treatments for pipe 

and culvert openings is State standard drawings for highway 

drainage and the Roadside Design Guide. Another potential source 

of information are the offices of the Local Technical Assistance 

Program (LTAP).

Maintaining Good Drainage at Intersections and Access 

Points

Access points, such as road intersections, driveways, pedestrian 

and bicycle crossings are important areas where drainage features 

should be reviewed and, where appropriate, improvements made.

At access points the grade of the highway and the access point 
23

have to meet. Consequently, the free flow of run-off is restricted 

and some drainage feature is usually built to move the water 

away. Drainage facilities, particularly those for driveways, may 

have been poorly constructed by developers or property owners 

and may be potential hazards. Local jurisdictions may have 

established laws requiring access points to be free of hazards and 

obstacles that could affect the safety of the traveling public.

 

In this photo, water is ponding 

because the outflow ditch has silted 

up. Ponded water can cause drivers 

to leave their lane, reduce braking 

ability or contribute to pavement 

deterioration.

This grated inlet can be 

used to prevent water 

ponding at access points.
24

V. WORKER SAFETY

The first consideration during any installation or maintenance 

of drainage features is the safety of both the work crew and the 

motorist. Therefore, remember to follow these safety procedures:

h Assure workers have proper Personal Protective 

Equipment.

h Before any excavation begins, determine if underground 

utilities are in the area.

h Follow safe trenching operation techniques.

h Comply with confined space requirements.

h Be aware of crushing hazards and pinch points.

h Be aware of loose materials, excavation drop-offs, tripping 

hazards, uneven ground.

h Avoid walking and working under suspended loads.

h Determine escape routes in case of emergency.

In addition, before work begins, it is important to set up proper 

work zone traffic control. Appendix B provides three typical traffic 

control plans that may be applicable depending upon the type of 

work being performed-- use layout no. 1 (6H-1) when the work is 

beyond the shoulder area; use layout no. 2 (6H-6) when the work 

is being done on the shoulder, and use layout no. 3 (6H-17), if the 

work is a moving operation such as for cleaning out curb inlets. 

These are for illustrative purposes; refer to the Manual on Uniform 

Traffic Control Devices for more detailed guidance.
25

VI. REFERENCES

1. American Association of State Highway and 

Transportation Officials. Roadside Design Guide. 2002 Third 

Edition, 2006 (with updated chapter 6). Washington, D.C. 

(www.transportation.org)

2. Federal Highway Administration. Manual on Uniform 

Traffic Control Devices. 2003 Edition. (http://mutcd.fhwa.

dot.gov/)
26

APPENDIX A: CLEAR ZONE DESCRIPTION

The concept of clear zone is an approach to minimize the number 

and severity of crashes involving vehicles running off the road or 

encountering slopes that may lead to instability and overturning. 

Simply stated, it is a traversable area that starts at the edge of the 

traffic lane and extends laterally a sufficient distance to allow a 

driver to stop or return to the road before encountering a hazard 

or overturning. The traversable area would be considered safe, 

if there were no fixed objects, unless they are breakaway, and 

if the roadside geometry (either the fore slope, back slope, or 

ditch) was flat enough that a vehicle could safely traverse the 

area without tipping and rolling over. Roadside safety features 

include breakaway sign and light posts, and traversable drainage 

structures. Curbs are not considered a roadside safety feature 

since they can be easily mounted by errant vehicles; hence, their 

presence does not alter how clear zone is measured.

A safe traversable slope can be either a recoverable slope or a 

non-recoverable slope with a clear run-out area at the bottom. A 

recoverable slope is a slope on which a motorist may, to a greater 

or lesser extent, retain or regain control of a vehicle and recover 

or stop. Slopes 1:4 (Vertical:Horizontal) or flatter are generally 
27

considered recoverable. A non-recoverable, traversable slope is 

a slope which is considered traversable but on which an errant 

vehicle will continue to the bottom. Embankment slopes from 1:3 

and 1:4 may be considered traversable but non-recoverable if they 

are smooth and free of fixed objects. A clear run-out area is the 

flatter area at the toe of a non-recoverable slope available for safe 

use by an errant vehicle. Slopes steeper than 1:3 are not considered 

traversable and should not be found in the clear zone. 

The objective of roadside safety is to provide and maintain 

as much clear zone as practical. The design clear zone is the 

minimum width to be provided on a project and is dependent 

upon speeds, the roadside geometry, and traffic volumes. Further 

details on clear zone can be found in the Roadside Design Guide.
28

APPENDIX B. THREE TYPICAL WORK ZONE 

LAYOUTS FROM MUTCD

1. Typical Traffic Control Layout for Work Beyond the Shoulder
29

2. Typical Traffic Control Layout for Shoulder Work with Minor 

Encroachment
30

3. Typical Traffic Control Layout for Mobile Operations on TwoLane Road

 
31

NOTES
32

NOTES
33

NOTES


Publication No. FHWA-SA-09-024

U.S. Department of Transportation

Federal Highway Administration

Office of Safety

1200 New Jersey Avenue, SE

Washington, D.C. 20590

 http://safety.fhwa.dot.gov/