7. Number of Lanes.

This is an unedited draft reflecting my personal opinions. Ezra Hauer

7. Number of Lanes. E. Hauer, Draft1, March 24, 2000 1981. Turner et al. analyse three years (1975-1977) of accident data (16334 accidents, 8815 nonintersection accidents) from 85 sites (1255 km) thought to be representative of two-lane roads without paved shoulders, two-lane roads with paved shoulders, and four-lane undivided roads without shoulders in Texas. These four-lane roads were referred to as ‘poor-boy’ roads since they were originally two-lane roads with full paved shoulders. The main results are presented in Figures 1 and 2.

Figure 1. All accidents

Figure 2. Non-intersection accidents

Whether one examines all accidents or only non-intersection accidents, it is evident that the accident rate is lowest on 2-lane roads with full paved shoulders and highest on two-lane roads without paved shoulders. The results are similar if one examines injury accidents, run-off-road, or multi-vehicle accidents. (It is not clear from the report whether ‘without paved shoulders’ means ‘no shoulder’.) One may conclude that two-lane roads that have ‘trait package A’, consisting of paved shoulders + all the other traits that go with this paved shoulders have a lower accident rate than, four-lane roads that have ‘trait package B” one of the traints being no paved shoulders. One may not conclude on the basis of this investigation that the differences in accident rate seen in Figures 1 and 2 can be 1

Earlier drafts of this papers were prepared in the course of a project for UMA Engineering (for the new Canadian Geometric Design Guide) and for DELCAN (in ORSAM 98). Number of lanes.wpd

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attributed to the presence or the absence of paved shoulders or to the difference in number of lanes.. Another complicating factor is that, in Texas, where a paved shoulder is available, it is common for slower vehicles to move to the shoulder to allow faster vehicles to pass. 1982. Rogness et al. in a before-after study, examined the safety effect of adding paved shoulders to roads two-lane rural roads with unpaved shoulders and of converting a two-lane with full shoulders cross-section to a four lane undivided road without shoulders. A total of sixty sites making up 394 miles was examined. The effect of conversion from two-lane sections with full shoulders into fourlane sections without shoulders for all accidents is shown in Figure 3.

Figure 3. Conversion to four-lane without shoulder (Poor Boy) The three points above the ‘equality line’ are for roads with ADT in the 1000-3000 range. The effect on non-intersection accidents is shown in Figure 4. Rogness at al. conclude that “The addition of full-width paved shoulders to a two-lane roadway was effective in reducing the total number of accidents that occurred.”...”When two-lane roadways with paved shoulders were converted to undivided, four-lane roadways without shoulders, ...., at low volumes (ADT 1000-3000) total accident frequency actually increased after the conversion. At moderate and high-volume locations, poor boy roadways resulted in fewer total Figure 4. Conversion to Poor Boy, nonintersection accidents. accidents.”

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Note that what Rogness et al. find by a before-after study squarely contradicts what Turner et al. (1981) find by a cross-section study for the same kind of roads. Were one to judge by Figures 19 and 20 it would seem that the Poor Boy roads have accident rates that are about twice those for two-lane roads with shoulders. 1986 Harwood compares the safety of the ‘two-lane undivided’ (2U) base condition in suburban conditions to various multilane design alternatives. In this context he assembled a data base that consists of five-year accident history for 469 miles of road for California and Michigan, with the average ADT around 15,000. After controlling for difference in several variables (driveways/mile, % trucks, intersections/mile) the resulting basic accident rates (accidents/MVM) for non-intersection accidents on suburban arterial highways are in Table 1. Table 1. Non-Intersection Accidents/MVM; Suburban Arterials. Type of development

Design alternative 2U*

3T**

4U

4D

5T

2.39

1.56

2.85

2.90

2.69

Residential 1.88 1.64 0.97 *2 lanes, undivided , **3 lanes with TWLTL

1.39

1.39

Commercial

It would appear then that there are more accidents on four-lane undivided roads than on two-lane undivided roads carrying the same traffic when in a commercial area, but fewer (about one half) if in a residential area. It would also appear that dividing a four lane road is to the detriment of safety. The results for unsignalized intersections are in Table 2. Table 2. Unsignalized-Intersection Accidents/MVM; Suburban Arterials. Type of development

Design alternative 2U*

3T**

4U

4D

5T

2.11

2.43

4.77

4.71

3.11

Residential 2.88 1.91 3.03 *2 lanes, undivided , **3 lanes with TWLTL

2.71

1.85

Commercial

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Here the indication is that four-lane undivided roads have (substantially) more accidents than two-lane roads. Dividing a four-lane road has a slight safety premium for unsignalized intersection accidents. Adding a TWLTL is of large benefit to unsignalized intersection accidents in three of the four cells (presumably because many left turns now take place elsewhere). The corresponding values for the sum of non-intersection and unsignalized intersection accident rates is in Table 3. Table 3. Non-intersection + Unsignalized-Intersection Accidents/MVM; Suburban Arterials. Type of development

Design alternative 2U*

3T**

4U

4D

5T

4.50

3.99

7.62

7.61

5.80

Residential 4.76 3.55 4.00 *2 lanes, undivided , **3 lanes with TWLTL

4.10

3.24

Commercial

It appears that four-laning in commercial areas is to the detriment of safety while doing so in a residential area benefits safety. In addition, dividing a four lane arterial does not benefit safety. 1988. Conversion from two to four-lanes and the addition of lanes in general is usually motivated by an increase in traffic and congestion. Projects of this kind have two kinds of effects. Effects felt locally, that is, along the stretch where capacity has been added, and effects felt elsewhere, which stem from the redistribution of traffic flow and from the shifting of bottlenecks to sites where capacity has not been added. Levine et al. discuss the issue in the context of lane additions on urban freeways. Two cases studies involving the addition of mixed flow lanes at the expense of the inside shoulder in Orange County, California, were undertaken. In one of the projects The number of accidents for about six miles upstream of the lane-addition section and half-way through the project remained roughly the same from ‘before’ to ‘after’. However, towards the end of the lane-addition section and for some six miles downstream there was a clear increase in accidents from before to after. In the second project the safety effect seems to be the opposite. Accidents increased (somewhat) upstream of the lane-addition section. The authors conclude that the safety effect of lane additions cannot be assessed by examining the change in the project area only and that “he most pronounced effect of the introduction of additional lanes was the shift of accidents from one location to another”.

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1995 b. Hadi et al used four years of Florida crash data to estimate NB models for nine road classes. Table 4 Road type

Miles

Two-lane rural

4203

Four-lane, urban, divided

1090

Freeway, rural

973

Four-lane, rural, divided

856

Two-lane, urban

618

Six-lane, urban, divided

345

Freeway, four-lane, urban

341

Freeway, six-lane, urban

162

Four-lane, urban, undivided

103

The model form eÓâ×covariate has been chosen. Whether a covariate was included amongst the regressors has been decided in the stepwise regression by the Akaike information criterion. A characteristic result is in Figure 5.

Because somewhat different covariates are used for each road type, and because the regression parameters vary,

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Figure 5

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comparisons of this kind depend on what values of covariates were chosen. Were a different number of intersections or median width chosen, the comparison would be different. 1998. Wang et al. examined the influence of cross-section elements on the safety of rural, multilane, non-freeway roads using HSIS and photolog data from Minnesota. Although they apply the model to compare the safety of a divided and undivided highway, in reality their model is based on 447 miles of four-lane divided and only 20 miles of four-lane undivided highways. 1999. Council and Stewart developed models to predict crashes/km-year for typical two-lane, fourlane undivided and four-lane divided roads. The basic traits of typical roads are in Table 5. Table 5 Two-lane

24 ft. paved travelway with 6 or 8 ft. shoulders and 22 ft. “ 6 ft. “

Four-lane, Undivided

48 ft. paved travelway with 8 ft. outside shoulders

Four-lane, divided

24 ft. paved travelway in each direction with 10 or 12 ft. shoulders and median from 16 ft. (raised) to 60 ft.

Data from California, Washington, Michigan and North Caroline served for analysis. The available miles of road by state and road type is in Table 6. Table 6. Miles of road in sample. Two-lane

Four-lane, Divided

Four-lane, Undivided

North Carolina

4900

325

insufficient data

Washington

1796

67

insufficient data

Minnesota

4370

414

insufficient data

California

3747

279

110

Only non-intersection and non-intersection-related crashes were used in the comparison. The model was of the form: crashes/km=Length×eâ0×ADTâ1×eâ2×shoulder width×eâ3×Surface width. The parameter estimates are in Table 7.

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Table 7. Parameter estimates. North Carolina Washington Minnesota California

2-lane 4-lane,divided 2-lane 4-lane,divided 2-lane 4-lane,divided 2-lane 4-lane,divided 4-lane, undivided

â0

â1

â2

â3

-2.9915 -4.6914 -6.2152 -4.5387 -8.1823 -7.2548 -3.0188 -8.9871 -8.7176

0.6725 0.7615 0.9669 0.6355 1.1758 1.0644 0.9048 1.0707 1.1213

-0.123 -0.2877 -0.4541

-0.1506

-0.2949 -0.2339 -0.3419

-0.4167

The resulting safety performance functions are shown in the four figures below.

Figure 6. North Carolina

Figure 7. Washington

Figure 8. Minnesota

Figure 9. California

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It appears that four-lane roads, when divided, have substantially fewer crashes. Note that the plots were produced assuming typical conditions. This meant a 1.83 m (6 ft.) shoulder on two-lane roads and a 3.05 m (10 ft.) shoulder on four-lane roads. Judging by the estimates of â2 in Table 7, were the shoulders of a two-lane road to be increased by 1.22 m, this alone would result in a 15%-55% reduction in crashes. In addition, two-lane roads were found to have a much higher driveway density than four-lane roads. This too may account for a part of the noted difference.

References Council, F. and Stewart, J. R. (2000) Safety effects of the conversion of two-lane rural to four-lane rural roadways based on cross-sectional models. Transportation Research Board Annual Meeting. Hadi, M. A., Aruldhas, J., Chow, L-F., and Wattleworth, J. A.,(1995), Estimating safety effects of cross-section design for various highway types using negative binomial regression. Transportation Research Record. 1500, 169-177 Harwood, D. W.,(1986). Multilane design alternatives for improving suburban highways. 282. National Cooperative Highway Research Program, Washington, D.C. Levine, D. W., Golob, T. F., and Recker, W. W.,(1988), Accident migration associated with lane-addition projects on urban freeways. Traffic Engineering and Control. 29, (12), 624-629 Rogness, R. O., Fambro, D. B., and Turner, D. S.,(1982), Before-after accident analysis for two shoulder ipgrading alternatives. Transportation Research Record. 855, Washington, D.C., 41-47 Turner, D. S., Fambro, D. B., and Rogness, R. O.,(1981), Effects of paved shoulders on accident rates for rural Texas highways. Transportation Research Record. 819, 30-37 Wang, J., Hughes, W. E., and Stewart, R.,(1998). Safety effects of cross-section desing on rural multi-lane highways. FHWA-RD-98-071. FHWA,

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