TECHBRIEF Assessing Pavement Surface 

Splash and Spray Impact on 

Road Users

 

 

FHWA Publication No.:FHWA-HRT-15-062 

FHWA Contact: Larry Wiser, HRDI-30, 202-493-3079

larry.wiser@dot.gov

This document is a technical summary of the unpublished 

Federal Highway Administration (FHWA) report, Splash and 

Spray Assessment Tool Development Program (FHWA 

Contract No. DTFH61-08-C-00030), available through the 

VTechWorks the Virginia Tech institutional repository, 

managed by the University Libraries,

https://vtechworks.lib.vt.edu/handle/10919/50550.

Objective

This TechBrief describes the development of an assessment 

tool to characterize the propensity of highway sections to 

generate splash and spray during rainfall and the impact of 

splash and spray on road users.

Introduction

The effects of vehicle splash and spray are well known to 

motorists who have driven in wet weather conditions. 

Research suggests that splash and spray contribute to a 

small but measureable portion of road traffic accidents and 

are sources of considerable nuisance to motorists. 

Furthermore, splash and spray from highway pavements can 

also carry a number of pollutants and contaminants. When 

deposited, these contaminants can be detrimental to plant life 

and accelerate the corrosion of roadway appurtenances.

Splash and spray are individually definable processes that 

are the products of a number of different factors. Many 

parties have gone to great lengths to reduce the splash and 

spray created by motor vehicles (especially heavy vehicles)

by retrofitting devices that alter the vehicle’s aerodynamics. 

Another possible solution to the problem is to change the 

characteristics of the highway pavement, such as pavement 

geometry, drainage, texture, and porosity.

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center 

6300 Georgetown Pike

McLean, VA 22101-2296

http://www.fhwa.dot.gov/

research/tfhrc/programs/

infrastructure/pavements/ltpp/
2

Research

The development of the assessment tool 

included a combination of laboratory and 

field experiments, theoretical 

developments, and computer simulations. 

The project started with an evaluation of 

prior work in the area of splash and spray 

mechanisms. This evaluation showed that 

(1) the main factors affecting splash and 

spray are water film thickness, vehicle 

speed, tire geometry, tire tread depth, 

vehicle aerodynamics, and vehicle spray 

suppression devices and (2) none of the 

techniques that have been used to 

measure splash and spray are widely 

accepted and readily available for routine 

pavement surface evaluations. The 

literature also suggested that there are

four prevailing splash and spray 

mechanisms: bow wave, side wave, tread 

pickup, and capillary adhesion.

The work then continued with the 

development of the three submodules that 

comprise the splash and spray 

assessment tool: (1) a water film 

thickness model, (2) an exposure model 

for estimating the amount of water that will

be projected by the tire, and (3) a splash 

and spray model that predicts the 

likelihood of splash and spray occurrence 

based on the other two models. 

The water film thickness model predicts

water film thickness on pavement 

surfaces based on pavement surface 

properties and rain intensity and is based 

on laboratory experiments. An indepth 

analysis of previous water film thickness

models allowed the development of a 

generic formula to compute the water film 

thickness as a function. Relevant 

parameters and values for the constants 

in the formula were determined using the 

results from flume experiments. 

Rainfall data available nationwide from 

the National Climatic Data Center were

processed to create an inventory of the 

expected percentage of time that the 

precipitation intensity would exceed 

various precipitation levels. Estimates for 

areas with weather stations were 

determined using a geographic 

information system-based spatial 

interpolation process. This inventory was 

incorporated into the exposure model.

The exposure model estimates the 

amount of water that will be projected by 

the tire given the water film thickness, 

pavement characteristics, and vehicle 

speed. The four prevailing splash and 

spray mechanisms—bow wave, side 

wave, tread pickup, and capillary 

adhesion—for different vehicle speeds 

and water film thicknesses were modeled 

using a computational fluid dynamic 

(CFD) model of splash and spray 

generation. This was done by simulating 

in a virtual wind tunnel the most critical 

maneuvers and vehicle combinations

identified in field experiments. For 

example, figure 1 compares the 

trajectories of the water droplets for two 

truck speeds.

Figure 1. Examples of CFD simulations.

V= 30 mi/h

V= 60 mi/h
3

The splash and spray model predicts the 

likelihood of splash and spray occurrence 

based on the other two models. The 

outcomes of the various simulation runs 

were synthesized in a set of simple 

equations that model the mechanisms 

and combine them into a simple equation 

to model pavement splash and spray.

Field tests conducted under controlled 

precipitation conditions for assessing the 

impact of splash and spray on road users 

allowed for the definition of a new 

measure of splash and spray: the 

occlusion factor. The occlusion factor was 

defined as the ratio of the mean 

luminance of the black squares to the 

mean luminance of the white squares on 

a large checkerboard placed on the 

splash- and spray-generating vehicle as 

captured by a camera placed on the 

following vehicle and pointed toward the 

board (figure 2). 

Figure 2. Occlusion factor definition.

A statistical analysis of the collected data 

showed that the occlusion factor 

correlated well with the five subjective 

user perception variables studied 

(obstruction, concentration, risk, 

confidence, and control) and thus can be 

used as a measure of the nuisance 

produced by splash and spray. 

Finally, the models were implemented in 

simple prototype tools and validated 

through field experiments and pilot 

applications using existing data. These 

pilot applications showed that the 

developed splash and spray assessment 

model was practical and could be used to 

support highway engineers’ decisions 

regarding highway design and 

maintenance. 

Conclusions 

The main product of the study is a model 

that can be used to predict splash and 

spray based on pavement surface 

characteristics and climatic conditions. 

The model is used as follows:

1. Compute the water film thickness 

based on the rainfall intensity and 

pavement surface properties.

2. Compute the maximum amount of 

water available for splash and spray.

3. Compute the contribution of each 

splash and spray mechanism until the 

total amount of available water is 

exhausted. 

4. Compute the spray density 

corresponding to each mechanism 

based on the corresponding mass flow 

and the speed of the vehicle.

5. Compute the total spray density.

6. Covert the spray density level to a 

subjective nuisance index (figure 3).
4

This model can provide useful information for supporting highway design and 

maintenance business processes.

Recommendations 

Future research or actions should include the following: 

 Investigate whether there are texture parameters that should be taken into 

account when calculating water depth.

 Investigate the impact of missing hours of precipitation recordings on the accuracy 

of the predictions. 

 Conduct additional verification of the mechanism modeled using CFD and take 

into consideration the distribution of water droplet size and longer experimental 

road segments.

Additional Information

For additional information, contact Larry Wiser at 202-493-3079 or the following 

address: 

Turner-Fairbank Highway Research Center, Office of Infrastructure Research and 

Development, 6300 Georgetown Pike, McLean, VA 22101-2296.

Figure 3. Example of splash and spray density estimation for a road segment.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

2 4 6 8 10 12 14 16 18 Spray Density kg/m^3)

Mile Post 

Splash & Spray Density under 0.68 inch/hour rain

1 2 3 4 5

Level of Nuisance
5

Researchers—This study was performed by the Center for Sustainable 

Transportation Infrastructure of the Virginia Tech Transportation Institute, Transport 

Research Laboratory, Applied Pavement Technologies, and the Virginia Center for 

Transportation Innovation and Research.

Distribution—This TechBrief is being distributed according to a standard 

distribution. Direct distribution is being made to the Divisions and Resource Center.

Availability— This TechBrief may be obtained from the FHWA Product Distribution 

Center by e-mail to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 

239-1160, or online at http://www.fhwa.dot.gov/research.

Key Words—Splash and spray, water film depth, pavement macrotexture, 

computational fluid dynamics (CFD), occlusion factor, user nuisance.

Notice—This document is disseminated under the sponsorship of the U.S. 

Department of Transportation in the interest of information exchange. The U.S. 

Government assumes no liability for the use of the information contained in this 

document. The U.S. Government does not endorse products or manufacturers. 

Trademarks or manufacturers’ names appear in this TechBrief only because they 

are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides 

high-quality information to serve Government, industry, and the public in a manner 

that promotes public understanding. Standards and policies are used to ensure and 

maximize the quality, objectivity, utility, and integrity of its information. FHWA 

periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.

 October 2015 FHWA-HRT-15-062

 HRDI-30/10-15(WEB)E