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Tree

Crashes

Roadway departures accounted for 57,475 highway deaths between 2016 and 2018. 

Trees were the most harmful event in 10,697 of these fatalities.

Hitting a tree1is the most harmful event in approximately 10 percent of all U.S. traffic fatalities each year and 19 percent 

of all roadway departure2(RwD) fatalities. There were 3,566 average annual RwD fatalities between 2016 and 2018

where a tree was identified as the most harmful event. Figure 1 shows the key characteristics where trees are 

overrepresented in crashes.

To reduce severe RwD crashes, FHWA recommends that State 

and local agencies consider three broad strategies:

1. Keep vehicles on the roadway.

2. Reduce the potential for crashes when vehicles leave the 

roadway.

3. Minimize the severity of crashes that do occur.

This brochure describes solutions that can be used to reduce 

fatal and serious injuries resulting from RwD crashes into 

roadside trees or shrubs for each of these three strategy areas

(Ref. Roadway Departure Safety). 

• Roadside Vegetation Control — Clearing roadside 

vegetation, particularly trees close to the road, can enhance a 

driver’s view of the roadway ahead, approaching vehicles, 

wildlife, and adjacent traffic control devices. In addition, 

reducing the shade from trees reduces ice on pavement and

provides better growth of roadside grasses to prevent erosion (Ref. FHWA-SA-07-018).

• Delineation Treatments — Chevron signs, curve warning signs, and sequential flashing beacons can be applied 

to curves to warn drivers that the roadway alignment is changing. Installing chevron signs at horizontal curves 

on rural two-lane undivided roads can produce a 16 percent reduction in non-intersection fatal and injury 

crashes and a 25 percent reduction in nighttime, non-intersection crashes of all severity types (Ref. FHWA-SA17-058, CMF Clearinghouse IDs 2438 and 2439). Installing new fluorescent curve signs or upgrading existing 

curve signs to fluorescent sheeting on rural two-lane undivided roads can result in approximately a 25 percent 

reduction in non-intersection fatal and injury crashes and a 34 percent reduction in nighttime, non-intersection 

crashes of all severity types (Ref. CMF Clearinghouse IDs 2433 and 2435). Widening edge lines, along rural twolane highways, from 4 inches to 6 inches can reduce non-intersection, non-winter fatal and injury crashes by 

approximately 37 percent (Ref. FHWA-HRT-12-048, CMF Clearinghouse ID 4737).

• Shoulder and Edge Line Rumble Strips — Two multistate studies and more than a dozen state studies report 

reductions in single-vehicle run-off-road crashes of 13 to 80 percent when milled rumble strips are installed on 

two-lane rural roads, with a widely accepted reduction of 17 percent for fatal and injury run-off-road crashes at 

1 Crashes where the most harmful event is an impact with a tree (FARS code 42). Shrubs (FARS code 41) are not included.

2 Crashes in which the vehicle crosses an edge line or center line, or otherwise leaves the traveled way.

Keeping Vehicles on the Roadway

Figure 1. Characteristics of RwD fatalities associated with tree 

crashes as the most harmful event — FARS data 2016–2018.
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rural freeway locations. A reduction of 36 percent can be expected for single-vehicle run-off-road injury crashes

on rural two-lane roadways (Ref. Rumble Strips and Rumble Stripes, CMF Clearinghouse IDs 3447 and 3454).

• Friction Treatment — For locations with a history of RwD crashes and where the pavement

surface is in good condition, high-friction surface treatment (HFST) is an effective solution

to help reduce crashes. Research conducted by the National Transportation Safety Board

and FHWA indicates that about 70 percent of wet pavement crashes can be prevented or

minimized by improved pavement friction. At least 41 states have applied HFST to at least

one project site. A research report published by FHWA shows HFST is estimated to reduce 

wet crashes by 83 percent and total crashes by 57 percent (Ref. FHWA-HRT-20-061).

• SafetyEdgeSM — Run-off-road crashes are twice as likely to result in a fatality when a near-vertical pavement

edge is exposed (Ref. SafetyEdge). Installing the SafetyEdgeSM during all paving or resurfacing projects allows 

drivers who drift off roadways to return to the pavement with much less potential to lose control. A five-state 

study showed the SafetyEdgeSM will reduce fatal and injury crashes by 11 percent (Ref. FHWA-SA-17-044, CMF 

Clearinghouse ID 9660).

• Widen Clear Zones by Removing/Relocating Trees — A clear zone is “the unobstructed, traversable3area

provided beyond the edge of the through traveled way for the recovery of errant vehicles.” Removing trees

within the clear zone and/or widening the clear zone are proven strategies to reduce RwD crashes (Ref. Clear

Zones). AASHTO’s Roadside Design Guide includes suggestions for clear zone width based on average daily

traffic, horizontal curve radius, design speed, and sideslopes. One Ohio study determined that the removal or

relocation of roadside fixed objects for all road types reduced injury crashes by 38 percent (Ref. CMF Clearinghouse IDs 1024 and 1044). Similarly, a Texas study determined that widening the rural two-lane roadway clear

zone from 10 to 40 feet resulted in a reduction in single vehicle run-off-road injury crashes of 32 percent at

tangent sections and 51 percent at horizontal curve locations (Ref. CMF Clearinghouse IDs 4127 and 4128).

• Maintenance to Control Vegetation Growth — Vegetation management techniques and planting guidelines can

help minimize the potential negative effects of roadside vegetation. The guidelines should consider and include

maintenance of mowing limits within the clear zone as an initial activity to minimize future trees and shrubs.

Tree removal should be prioritized at critical locations such as where trees are located close to the road at

curves and intersections (Ref. FHWA-SA-07-018).

• Shielding Trees — Shielding trees by constructing longitudinal

barrier, such as guardrails, can reduce the severity of RwD crashes

when is it not practical to remove roadside trees. Adding a barrier

will likely result in more reported crashes because the barrier is

closer to the traveled way and is a bigger target than the obstacle

that it is shielding. For example, a Florida study determined that the

total number of crashes for principal arterials other freeways and

expressways increased by 6 percent with the installation of

guardrail, but the fatal and severe injury crashes reduced by 16

percent (Ref. CMF Clearinghouse IDs 8391 and 8393).

For additional information, contact Joseph Cheung, FHWA Office of Safety, at joseph.cheung@dot.gov; or Tori Brinkly, 

FHWA Resource Center, at victoria.brinkly@dot.gov.

Publication Number FHWA-SA-21-022

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Foreslopes between 1V:3H and 1V:4H may be considered traversable but non-recoverable if smooth and free of fixed objects.

Reducing the Crash Potential When Vehicles Leave the Roadway

recovery/regaining the roadway

Minimizing the Severity of Crashes

For Additional Information

Guardrail shielding large tree. Source: FHWA

Roadway Departure 

Safety – FHWA 

Office of Safety