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Publication Number: FHWA-RD-01-103
Date: May 2001

Highway Design Handbook for Older Drivers and Pedestrians

III. ROADWAY CURVATURE AND PASSING ZONES

 

Background and Scope of Handbook Recommendations

Crashes on horizontal curves have been recognized as a considerable safety problem for many years. Crash studies indicate that roadway curves experience a higher crash rate than tangents, with rates ranging from one-and-a-half to three to four times higher than tangents (Glennon, Neuman, and Leisch, 1985; Zegeer, Stewart, Reinfurt, Council, Neuman, Hamilton, Miller, and Hunter, 1990; Neuman, 1992). Lerner and Sedney (1988) reported anecdotal evidence that horizontal curves present problems for older drivers. Also, analyses of crash data in Michigan found that older drivers were involved in crash situations on horizontal curves as a result of driving too fast for the curve or, more significantly, because they were surprised by the curve alignment (Lyles, Kane, Vanosdall, and McKelvey, 1997). In reviewing literature on driver behavior on rural road curves, Johnston (1982) reported that horizontal curves that are less than 600 m (1968 ft) in radius on two-lane rural roads, and those requiring a substantial reduction in speed from that prevailing on the preceding tangent section, were disproportionately represented among crash sites.

Successful curve negotiation depends on the choice of appropriate approach speed and adequate lateral positioning through the curve. Many studies have shown that loss-of-control crashes result from an inability to maintain a lateral position through the curve because of excessive speed, with inadequate deceleration in the approach zone. These problems, in turn, stem from a combination of factors, including poor anticipation of vehicle control requirements, induced by the driver's prior speed, and inadequate perception of the demands of the curve.

Many studies report a relationship between horizontal curvature (and the degree of curvature) and the total percentage of crashes by geometric design feature on the highways. The reasons for these crashes are related to the following inadequate driving behaviors:

  • Deficient skills in negotiating curves, especially those of more than 3 degrees (Eckhardt and Flanagan, 1956).
  • Exceeding the design speed on the curve (Messer, Mounce, and Brackett, 1981).
  • Exceeding the design of the vehicle path (Glennon and Weaver, 1971; Good, 1978).
  • Failure to maintain appropriate lateral position in the curve (McDonald and Ellis, 1975).
  • Incorrect anticipatory behavior of curve speed and alignment when approaching the curve (Messer et al., 1981; Johnston, 1982).
  • Inadequate appreciation of the degree of hazard associated with a given curve (Johnston, 1982).

With respect to vertical curves, design policy is based on the need to provide drivers with adequate stopping sight distance (SSD). That is, enough sight distance must exist to permit drivers to see an obstacle soon enough to stop for it under some set of reasonable worst-case conditions. The parameters that determine sight distance on crest vertical curves include the change of grade, the length of the curve, the height above the ground of the driver's eye, and the height of the obstacle to be seen. SSD is determined by the driver's reaction time, speed of the vehicle, and tire-pavement coefficient of friction. There is some concern with the validity of the SSD model that has been in use for more than 50 years, however. Current practice assumes an obstacle height of 150 mm (6 in) and a locked-wheel, wet-pavement stop (AASHTO, 1994). Minimum lengths of crest vertical curves are based on sight distance and driver comfort. These criteria do not currently include adjustments for age-related effects in driving performance measures, which would suggest an even more conservative approach. At the same time, the general lack of empirical data demonstrating benefits for limited sight-distance countermeasures has led some to propose liberalization of model criteria, such as obstacle height (Neuman, 1989; Fambro, Fitzpatrick, and Koppa, 1997).

Standards and criteria for sight distance, horizontal and vertical alignment, and associated traffic control devices are based on the following driver performance characteristics: detection and recognition time, perception-reaction time, decision and response time, time to perform brake and accelerator movements, maneuver time, and (if applicable) time to shift gears. However, these values have typically been based on driving performance (or surrogate driving measures) of the entire driving population, or have been formulated from research biased toward younger (college age) as opposed to older driver groups. The models underlying these design standards and criteria therefore have not, as a rule, included variations to account for slower reaction time or other performance deficits consistently demonstrated in research on older driver response capabilities. In particular, diminished visual performance (reduced acuity and contrast sensitivity), physical capability (reduced strength to perform control movements and sensitivity to lateral force), cognitive performance (attentional deficits and declines in choice reaction time in response to unpredictable stimuli), and perceptual abilities (reduced accuracy of processing speed-distance information as required for gap judgments) combine to make the task of negotiating the highway design elements addressed in this section more difficult and less forgiving for older drivers.

This section will provide recommendations to enhance the performance of diminished-capacity drivers as they negotiate roadway curvature and passing zones, focusing on four design elements: A. pavement markings and delineation on horizontal curves; B. pavement width on horizontal curves; C. crest vertical curve length and advance signing for sight-restricted locations; and D. passing zone length, passing sight distance, and passing/overtaking lanes on two-lane highways.

Recommendations by Design Element

  A. Design Element: Pavement Markings and Delineation on Horizontal Curves
MUTCD:4

(1) Recommendations for the maintained brightness of white edgelines on horizontal curves are presented in terms of measured* effective luminance contrast level (C), where:

Luminance contrast (C) =
Luminancestripe - Luminancepavement

Luminancepavement

* See advisory comments pertaining to luminance measurement in Recommendation IC (2).

Specifically,

(1a) On highways without median separation of opposing directions of traffic, the recommended minimum in-service contrast level for edgelines on horizontal curves is 5.0.

(1b) On highways where median barriers effectively block the drivers' view of oncoming headlights or where median width exceeds 15 m (50 ft), the recommended minimum in-service contrast level for edgelines on horizontal curves is 3.75.

MUTCD:4

(2) For horizontal curves with radii less than 1000 m (3280 ft), it is recommended that standard centerline markings be supplemented with raised pavement markers (RPM's) installed at standard spacing (i.e., 12 m [40 ft] apart), and that they be applied for a distance of 5 s of driving time (at 85th percentile speed) on the approach to the curve and continued throughout the length of the curve. [See time-speed-distance table on page 3.]

MUTCD:4

(3) In addition to the installation of chevron alignment signs (W1-8) as specified in section 2C.10 of the MUTCD (FHWA, 2000), it is recommended that:

(3a) Roadside post-mounted delineation devices (PMD's) be installed at a maximum spacing (S) of 12 m (40 ft) on all horizontal curves with a radius (R) of 185 m (600 ft) or less.

(3b) The standard formula specified in MUTCD section 3D.4, Table 3D-1 (FHWA, 2000) be used to define roadside delineator spacing intervals for curves of radii more than 185 m (600 ft), where:

English:
S= 3 square root of R -50
Where: R=radius of curve (in feet)
S=spacing on curve (in feet)
Metric:
S = 1.7 square root fo R - 15
R=radius of curve (in meters)
S=spacing on curve (in meters)

The rationale and supporting evidence for these recommendations

 

  B. Design Element: Pavement Width on Horizontal Curves
AASHTO:4
ITE:4

(1) For horizontal curves on two-lane non-residential facilities that have 3 degrees of curvature, it is recommended that the width of the lane plus the paved shoulder be at least 5.5 m (18 ft) throughout the length of the curve (assuming AASHTO [1994] design values for superelevation and coefficient of side friction).

The rationale and supporting evidence for this recommendation

 

  C. Design Element: Crest Vertical Curve Length and Advance Signing for Sight-Restricted Locations
MUTCD:1 (1) To accommodate the exaggerated decline among older drivers in response to unexpected hazards, it is recommended that the present criterion of 150 mm (6 in) for obstacle height on crest vertical curves be preserved in the design of new and reconstructed facilities.
IEC: requires
FHWA permission
(2)SLOW Hill Blocks View sign

Where a need has been determined for installation or replacement of a device to warn motorists that sight distance is restricted by a crest vertical curve, the message SLOW / HILL BLOCKS VIEW is recommended, using the special sign size of 900 mm x 900 mm (36 in x 36 in) as a minimum.

MUTCD:4 (3) If a signalized intersection is obscured by vertical or horizontal curvature in a manner that the signal phase becomes visible at a preview distance of 8 s or less (at operating speed), then it is recommended that the standard (W3-3) advance signal warning sign be augmented with a yellow placard bearing the black legend PREPARE TO STOP and a flashing yellow beacon interconnected with the traffic signal controller. The yellow flasher should be activated at a sufficient interval prior to the onset of the yellow signal phase and sustained after the onset of the green signal phase to take into account the end of queues experienced during peak traffic conditions, as determined through engineering study. [See time-speed-distance table on page 3.]

The rationale and supporting evidence for these recommendations

 

  D. Design Element: Passing Zone Length, Passing Sight Distance, and Passing/Overtaking Lanes on Two-Lane Highways
AASHTO:2
ITE:2
MUTCD:2
(1) To accommodate age-related difficulties in judging gaps and longer decision-making and reaction times exhibited by older drivers, the most conservative minimum required passing sight distance (PSD) values, as determined by AASHTO (1994, table III-5), are recommended.
MUTCD:1 MUTCD:3 (2) Use of the MUTCD (FHWA, 2000) special-size (1200-mm x 1600-mm x 1600-mm [48-in x 64-in x 64-in]) NO PASSING ZONE pennant (W14-3), or the standard size (900 mm x 1200 mm x 1200 mm [36 in x 48 in x 48 in]) using fluorescent yellow retroreflective sheeting, as a high-conspicuity supplement to conventional centerline pavement markings at the beginning of no passing zones is recommended.
AASHTO:4
ITE:4

(3) To the extent feasible for new or reconstructed facilities, the implementation of passing/overtaking lanes (in each direction) at intervals of no more than 5 km (3.1 mi) is recommended.

The rationale and supporting evidence for these recommendations

 

FHWA-RD-01-103

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