Low Cost Treatments for Horizontal Curve Safety
CHAPTER 4. OTHER TRAFFIC CONTROL DEVICE TREATMENTS
REFLECTIVE BARRIER DELINEATION
Retroreflective material, such as reflectors or panels of retroreflective sheeting, can be a highly effective treatment for delineating curves, especially at nighttime. Strips of reflective sheeting shaped to provide linear reflectorization are applied to either concrete barriers or metal guardrail to alert drivers of approaching curves. The example shown below consists of 34-in-long, 4-in-wide to 6-in-wide panels. The color of the delineation should be the same as traditional delineation—the same color as the adjacent edge lines. On a two-lane, two-way road, this means that the delineation would be white on both sides of the road.
Reflective sheeting shaped to provide linear reflectorization.
The photographs below show the resulting linear reflectorization effect on construction zone barriers. Agencies can achieve similar effective delineation on rural curves by applying the reflective panels to existing metal guardrails.
There are no published guidelines for when this or similar treatments are appropriate. Obviously, an existing guardrail or concrete barrier must be in place before any material is applied. When the existing barrier is frequently hit consider adding new or supplemental reflectorization.
Space reflective sheeting panels 18 to 36 in apart, running parallel to traffic. Attach the panels to concrete barriers by anchoring them into drilled holes supplemented with a caulking compound. To apply on metal guardrail, adhesive is used to attach the sheeting panels, housed in brackets, to the guardrail.
The photos below illustrate reflector application on metal guardrail. Individual reflectors can be spaced according to the criteria noted for post delineators, discussed earlier. For curved sections of guardrail, it is important to adjust angles of individual reflectors (i.e., bend mounting brackets) to ensure their alignment is perpendicular to the angle of oncoming headlights.
Continuous and intermittent application of retroreflective sheeting on metal guardrail.
Installation of reflectors on W-beam guardrail.
Oregon DOT (ODOT) conducted an example application of reflective barrier treatments (shown above) known as the 3M Linear Delineation System. Results reveal that the retroreflective panels provide a good alternative to traditional concrete barrier delineation methods (such as reflective barrier markers). ODOT realized at the end of construction that the panels could be removed from the barrier for reuse on future projects. The success of the panels has led ODOT to consider future implementation when crash histories show the need for additional safety measures. However, installing the panels proved to be more challenging then envisioned, primarily because it is a time-intensive process. The evaluation also cited maintenance concerns about keeping the panels clean from dirt and road grime to maintain an optimal retroreflectivity level.
Individual reflectors for guardrail application cost approximately $3.00 each. Strips of reflective sheeting, known as the guardrail Linear Delineation System, are available at approximately $2.33/lineal ft of 4-in-wide white material.
ROADSIDE OBJECT DELINEATION
Trees, utility poles, and other obstructions located in the roadside area close to the nearest travel lane (such as within the designated clear zone) can be run-off-road road crash hazards. Installing an object marker, reflectorized tape, or other simple delineation device is a low-cost procedure to reduce the associated crash potential.
Depending on the roadside obstruction location and the severity of the hazard, agencies should first consider removing objects that have a history of being struck. Agencies will also discover the more difficult challenge of overcoming public resistance to removing trees. Other options to consider, such as modifying the roadside to shield the object from being struck, are flattening or grading sideslopes, regrading ditch sections, or improving shoulders. When cost rules out these alternatives, delineation is the preferred option. Regardless of the option selected, agencies must first install acceptable delineation on the curve, as previously discussed.
Simply adding a reflective tape to a pole or tree that cannot be removed may prevent a crash.
Once the decision is made to delineate a tree or other object, install a Type 2 or Type 3 object marker (see MUTCD Sections 3C.01 and 3C.04) according to the following principles:
The Pennsylvania Department of Transportation (PennDOT) has an experimental program targeting delineation to potentially hazardous objects on road segments with high run-off-road utility pole- and tree-related crash frequencies (particularly at night). The program is proving effective when it is not feasible to remove or relocate an object because of budget constraints or the object is on private property. PennDOT marks the tree or utility pole with a round of reflective tape. One round is used on each tree and each utility pole. Two rounds are used for poles at intersections. No evaluation has yet documented the effectiveness of this practice.
Contact and Further Information
A.J. Zeigler, Guide to Management of Roadside Trees, Report FHWA-IP-86-17, Michigan Department of Transportation, Lansing, MI 48909, December 1986.
Pennsylvania DOT Bureau of Highway Safety, Phone 717-705-1706.
Highway Safety and Trees: The Delicate Balance (FHWA-SA-06-13) is available as a DVD; email requests to Report.Center@dot.gov
DYNAMIC CURVE WARNING SYSTEM
Agencies can enhance curve warning systems by using supplemental beacons and/or messages that activate when a motorist approaches the curve at a high speed. A typical dynamic curve warning system combines a speed measuring device (such as loop detectors or radar) with flashing beacon and a variable message sign. The system is designed to slow high-speed vehicles as they approach and enter a horizontal curve. It works by measuring the speeds of approaching vehicles and providing messages to speeding drivers to slow down to an advisory speed. Agencies can develop these systems using off-the-shelf technology. The advantage of this treatment is that the device has a much greater effect on high-speed vehicles than a static curve warning sign. A variety of these systems are deployed in the U.S., as the three examples below demonstrate.
Speed Actuated Sign – Augusta, ME.
Flashing Beacon on Warning Sign.
(Courtesy of Caltrans)
Dynamic Warning System on I-80 in California.
Because even the least expensive system is much more costly than static signs, agencies should limit their application to locations with high crash rates, especially those involving fatalities and injuries, and where other less expensive devices have failed to solve the problem. The only specific guideline found for when to use such a device is from a study of a similar device, which recommended that ". . . a vehicle-actuated curve speed control may be required if there have been 10 or more reported accidents in a 24-month period, or 7 or more reported accidents in a 12-month period. The accidents should occur along the section of road including the curve and a distance of 1000 ft downstream."
One dynamic system application that does not require major reconstruction involves a radar speed detection device coupled with warning signs and activated flashing beacons. The Texas system, illustrated in the photos below, advises drivers detected driving more than 5 mph over the 25-mph curve advisory speed limit to reduce their speed. A radar detector measures speeds and displays them using a speed display sign stating: "YOUR SPEED IS . . .". A W1-1 warning sign is located 625 ft in advance of the curve, and the overhead sign is located in the point of curvature. The radar is set to start processing the speed data about 300 ft before a vehicle reaches the overhead sign.
Dynamic curve speed warning system in Camp County, Texas.
The cost of these systems varies, depending upon the specific design. The cost of the Texas system presented above, which consists of radar speed detection and associated activation of flashing beacon, was approximately $18,000 for equipment and installation. The system deployed on a California interstate was $61,000 (in year 2001; the cost included traffic control).
Dynamic curve warning systems can effectively reduce vehicle speeds in horizontal curves, especially during wet-pavement conditions. The accident effectiveness will likely vary by location. For the system displayed on a California interstate, Caltrans reported a 44 percent reduction in accidents in the first year and a 39 percent in the second year, compared to the year before installation.
Contacts and Further Information
For the California installation—Robert Peterson, Chief Traffic Safety Branch, District 3, Caltrans, Robert_Peterson@dot.ca.gov
SPEED LIMIT ADVISORY MARKING IN LANE
Pavement markings in advance of horizontal curves provide highly conspicuous, supplementary warning information and the potential to increase safety. In particular, advisory speed warnings provide essential information directly related to drivers’ safe negotiation of curves. Pavement markings are especially important for reducing speeds at curve locations where signs have proved ineffective.
An example speed-limit advisory pavement marking is illustrated at right. In this example, the markings display is CURVE—55—MPH. PennDOT has also experimented with the use of the pavement arrow (such as MUTCD figure 3B-21), as discussed in chapter 7. That arrow is currently reserved for use in the designating lane-use at intersections, so other uses such as for curves requires FHWA experimentation approval.
There is no established guideline for when to use this treatment. It is probably more appropriate for higher speed roads and those where speed studies indicate excessive speeding.
The MUTCD and the Standard Highway Signs (SHS) manual present specifications for designing and placing speed limit advisory pavement markings. MUTCD section 3B.19 has examples of elongated pavement marking letters and words suitable for reading by approaching motorists. Visit http://mutcd.fhwa.dot.gov/kno-2003r1.htm. The SHS includes layouts for applying these markings, including pavement markings. Visit http://mutcd.fhwa.dot.gov/ser-shs_millennium.htm.
The advance distance at which such markings are applied depends on both the approach speed and design speed of the curve. Agencies should base advance placement distances on specific approach and curve speeds, which should be the same as advance distances prescribed for warning signs, as provided in chapter 3.
A Texas evaluation of advisory speed pavement markings based its findings on speed reductions at curves compared with an upstream speed control point. While there were inconsistent results between data collection sites, the study concluded that the markings were "worthy of further exploration."
Speed-limit advisory pavement marking.
The SHS provides specifications for sign sizes and message layout.
Roadside Design: Steel Strong Post W-beam. A guidance memo was issued on May 17, 2010 on the height of guardrail for new installations. Guidance regarding existing guardrail will be developed in the next several months, in consultation with AASHTO’s Technical Committee on Roadside Safety.