Transportation agencies in the United States typically use circular curves at roadway horizontal curve transition locations. A motorcyclist adapts to the roadway curvature by reducing speed upon entry to the curve. This speed change is often accompanied by a path correction where the motorcycle may enter the curve on the outside of a travel lane, shift to the inside of the lane as the vehicle traverses the curve, and then return to the outside of the lane as the vehicle exits the curve. This path correction often occurs at the same time as the motorcycle accelerates for the length of the curve. Drivers of four-wheel vehicles make similar adjustments to their travel path.

Locations with identical radii, roadways with longer curves, larger AADT values, and isolated curve configurations (i.e., curves not located within 300-ft of another curve end) are stronger candidates for the placement of motorcycle-tobarrier crash countermeasures.

When compared with flat curves (radius greater than or equal to 4,000-ft), sharp curves with a radius less than 1,500-ft could be expected to increase the probability of a fatal or serious injury crash by approximately 8 percent. Moderate curves did not have any significant trend. Locations with reverse curves were affiliated with an increase of approximately 6 percent in fatal or serious injury crashes.

Risk for single-motorcycle crashes decreased as the horizontal curve radius increased. This equated to a motorcyclist having almost five times the risk of a crash on horizontal curves with a radius of 1,500-ft or less compared to risk on a similar road with a straight section. This amount reduced to about two times the risk for moderate curve radii (greater than 1,500-ft but less than or equal to 3,000-ft). For flatter curves with radii greater than 3,000-ft, the risk was 1.88 times that observed for a straight roadway segment. Researchers define a straight segment as one with a radius greater than 20,000-ft.

The placement of reverse curves resulted in more crashes than at locations without reverse curvature. This observation reversed the findings from the 2017 study by the same core group of authors. The authors hypothesized that this reversal was due to inclusion of an interaction term in the statistical model that accounted for curve radius as well as curve type.

Higher speed limit and presence of auxiliary lanes and intersections were all associated with higher crash frequencies. The researchers also determined that paved roads and shoulders were associated with higher speeds and therefore increased the likelihood of crashes. In contrast, they observed that narrower roads tended to be associated with fewer crashes. The researchers hypothesized that this observation may be because narrower roads tend to be lower-performing local roads and more riders on these facilities may be local riders who are familiar with the narrower road.

General equation except for rural two-lane:

Empirical CMFs specific to rural two-lane:

The main challenge for motorcyclists at intersections is motorists failing to register an approaching motorcyclist. This is often because of the small frontal aspect that a motorcyclist presents head on and other road users’ poor judgement of motorcycle approach speeds.

Treatment

: To reduce the risk to motorcyclists, clear sight distance needs to be maintained at intersections so that driver sight lines are optimized, and motorcyclists do not become ‘masked’ by roadside features. This includes restricting parking on the approaches to intersections and positioning road furniture/vegetation out of the sight triangle.

Inter-visibility between drivers and motorcyclists can be improved by the minor approach of an intersection being perpendicular to the major road. This also helps to reduce the likelihood of the view of the driver being obscured by the vehicle’s door pillar and consequently increasing the risk that the driver fails to see an approaching motorcycle.

Roundabouts can present a number of risks for motorcyclists since accelerating, braking and maneuvering are all necessary. As highlighted earlier within the chapter, this can be compounded when there are large amounts of pavement markings and metal plates in the pavement which can introduce variations in skidding resistance.

Collisions at roundabouts typically involve:

Drivers failing to yield;
Drivers looking but failing to see a motorcyclist;
Misjudgment of motorcyclist’s speed; and
Tight curves on entry.

Treatment: Motorcycle collisions at roundabouts can be partly mitigated by clear signing and markings, by limiting visibility to the left until 50-ft before the entry using suitable screening ( ), and by ensuring that the lane assignments of a multilane roundabout are self-explanatory.

Designers should avoid sharp curves on roundabout approaches and entries as these can cause loss of control collisions for motorcyclists. Reverse curves intended to calm traffic should be designed to keep side forces minimal. Tangent sections between consecutive horizontal curves of 75-ft or greater length are recommended to allow a motorcyclist to regain balance before navigating the second horizontal curve. Designers should also ensure that surfacing materials and pavement markings have suitable skidding resistance, especially at the crosswalks and yield entry lines where YIELD words, yield symbols, and dashed lines will be present to delineate the yield point at a roundabout entry.

The is the standard for traffic control devices on roads open to public travel. See Section 2C.33 for warning signs and plaques to be used for motorcyclist. Conditions that these signs came be used to mitigate include:

Some signs and plaques have special word message signs for situations where roadway conditions make it necessary to provide road users with additional warning information and where a standard message is not provided in the . As an example, these are the STEEL GRID DECK (W8-V1), EXPANSION JOINTS (W8-V2), and OPEN JOINTS ON BRIDGE (W8-V3) warning signs.

The need for smooth riding surfaces for motorcyclists should be provided, or advance warning notification given whenever roadways surfaces are disturbed. When plans call for milled or grooved pavement, the specifications should be reviewed so that the resulting surface is fine enough to prevent motorcycles tires from being captured or redirected at high speed.

The special needs and control of motorcyclists should also be considered through a temporary traffic control zone.

Pavement scarring of the roadway exceeding 1/8-inch should be repaired to prevent deterioration of the pavement surface and to provide a smooth surface for motorcyclists.

Metal coverings provide a challenge to motorcyclists when they traverse them unexpectedly in the roadway.

The design of the crossover should accommodate all vehicular traffic including motorcycles, trucks, and buses.