4.3.1 Control delay: comparing before and after solutions

Before redesigning the geometric configuration of the area of intersection to include a roundabout, for which the investigated database showed that the mean crash rate over 5 years was lower than at non-circular intersections, it was decided to adjust the current non-circular intersection in line with the requirements of the Italian Standard in force in two phases:

a.Phase I: adjusting the radius of the edges of the entry and exit legs of the intersection, the width of the traffic lanes, the removal of obstacles in the areas within the so-called sight triangles, and the addition of median-refuge islands on the major and minor roads, as recommended by the Italian Road Design Standard [14];

Exhibit 17-2 of the HCM2016 [13]. Table 5 shows the dj values for the intersections investigated as listed in Tables 2 and 3 and the corresponding crash costs from the cost of an injured person approximately equals to 73,631 Euro, fatality equals to

A 14 B—vehicle control delay 10–15 s 251,637 B 26 D—vehicle control delay 25–35 s 363,698 C 33 D—vehicle control delay 25–35 s 642,850 D 17 C—vehicle control delay 15–25 s 340,640

Overview of control delay and crash costs at non-circular intersections with the old configuration.

Table 5 shows that the total crash cost (TCC) increases when the control delay of the entire intersection dentire intersection investigated increases: when the mean dj of the vehicles leaving the intersection area increases, this indicates that drivers do not feel safe to make the maneuver. This circumstance is mainly due to the poor geometric configuration of the intersection and, as confirmed by the preliminary

Figure 7 shows a positive linear relationship using the ordinary least square method to predict the total crash cost in euros by varying the average control delay at the entire intersection, which is a measure of LoS (Eq. (9)). The parameters

1,394,434 Euro, and damaged vehicles almost of 7686 Euro.

Observed tc,j and tf,j values for all maneuvers at intersection A.

Transportation Systems Analysis and Assessment

(dentire intersection)

Study intersection Control delay

Figure 6.

Table 4.

Table 5.

172

results of this study, can amplify the frequency and severity of crashes.

Example of tc,j assessment using the Drew-Dawson method for two maneuvers at intersection A.

tc,j XYZ tf,j XYZ X — <1 s <1 s X — <1 s <1 s Y <1 s — 3.28 s Y <1 s — 1.76 s Z 6.62 s 4.46 s — Z 3.87 s 2.53 s —

Level of Service (LoS) Total crash cost (TTC)

s/veh EUR

average control delay for the roundabout approach as a whole in order to make comparisons with other intersection types, control delay dj for the ith approach was calculated by computing a weighted average of the delay for each lane on the approach (Eq. (7)), weighted by the volume in each lane. The calculation is shown in Eq. (10) using SETRA diagrams for the expected control delay at each maneuver,

Analytical Assessment of Effective Maintenance Operations on At-Grade Unsignalized…

dapproach,<sup>i</sup> <sup>¼</sup> dleft lane � qleft lane <sup>þ</sup> dright lane � qright lane

The control delay dentire roundabout for the entire roundabout is similarly calculated by computing a weighted average for the delay at each approach, weighted by the

dentire  roundabout <sup>¼</sup> <sup>∑</sup>dapproach,i qi

where dentire roundabout is the control delay for the entire roundabout, s/veh; dapproach,k is the control delay for approach kth, s/veh; qi is the flow rate for approach

4.3.2 Control delay at the entire intersection: comparing before and after solutions

the shape of a regular intersection but adjusting it to the Italian Standard (see Figure 8a and Section 4.3.1) was confirmed by the expected crash frequency values computed by adopting the SPF available in Biancardo et al. [16]. Biancardo et al. [16] worked in line with HSM2010 [17] procedure and revised the equation available in the Manual to predict crash frequency at three and four-leg rural

The Nspf formulation [16] was here used (see Eq. (12)) to predict the crash frequency at the two-lane two-way four-leg intersections studied in greater depth (intersection D) as it was calibrated using a data set that adequately reflects and partly overlaps with what is explored here. MLW is the mean lean width of the

Eq. (12) applies to an AADTmaj range from 0 to 14,700 vpd and AADTmin range

In HSM2010 [17], Crash Modification Factors (CMFs) are introduced to account for the specific site conditions that differ from the hypothesized base conditions. Under base conditions, the CMF is 1.00 (i.e., Figure 5d), while the CMF is less than 1.00 when a geometric configuration in compliance with the Standard and with many additional modules exists and, consequently, a reduction of average yearly crash frequencies can be expected. Npredicted (predicted average crash frequency for a specific year for site type x) is shown in Eq. (13), where the effect of the skew angle does not appear, as study intersection D has an 80° angle, very close to orthogonal road axes and, consequently, no additional benefits can derive from further correction or the right-turn lanes that already exist on major roads.

Nspf ¼ AADT � exp ½ � �1:042 � MLW � 8:5 (6)

It is imperative for a designer to understand the relationships between design

The effectiveness of all the changes that have been designed without changing

qleft lane þ qright lane

∑qi

(4)

(5)

as the calibration conditions reflect the actual study context.

DOI: http://dx.doi.org/10.5772/intechopen.86435

volume on each approach and represented by Eq. (11):

i th, vph.

features and crash frequency.

unsignalized at-grade intersections.

approaching and departing lanes.

from 0 to 3500 vpd.

175

#### Figure 8.

Advanced geometric design solutions for an existing non-circular intersection. (a) Adjustment according to the Italian Standard. (b) Changing the configuration into a compact roundabout (Dext = 26 m) according to the Italian Standard.

b.Phase II: adding left-turn lanes on major-road approaches based on the ratio between the volume of vehicles turning left per hour and the total traffic volume on the highway per hour.

Right-turn lanes on minor road stops are not permitted by the Italian Design Standard [14], and they are not included in the design.

The intersections investigated are almost totally equipped with right-turn lanes on major roads and they have lighting within and approaching the intersection area; consequently new lighting and new right-turn lanes on major roads are not required. Control delay at the entire intersection for the first advanced geometric solution of intersection D (Figure 8a) was estimated as in Section 3.2; the expected LoS of this geometric adjustment is shown in Table 7. In particular, to compute the


\*Adjusted to the Italian Standard without changing the shape by adding further modules Figure 8a. \*\*Figure 8b.

#### Table 7.

Comparison of the control delays at advanced geometric solutions with those of intersection D (Figure 4, old configuration).

Analytical Assessment of Effective Maintenance Operations on At-Grade Unsignalized… DOI: http://dx.doi.org/10.5772/intechopen.86435

average control delay for the roundabout approach as a whole in order to make comparisons with other intersection types, control delay dj for the ith approach was calculated by computing a weighted average of the delay for each lane on the approach (Eq. (7)), weighted by the volume in each lane. The calculation is shown in Eq. (10) using SETRA diagrams for the expected control delay at each maneuver, as the calibration conditions reflect the actual study context.

$$\mathbf{d}\_{\text{approxach, i}} = \frac{\mathbf{d}\_{\text{left lane}} \times \mathbf{q}\_{\text{left lane}} + \mathbf{d}\_{\text{right lane}} \times \mathbf{q}\_{\text{right lane}}}{\mathbf{q}\_{\text{left lane}} + \mathbf{q}\_{\text{right lane}}} \tag{4}$$

The control delay dentire roundabout for the entire roundabout is similarly calculated by computing a weighted average for the delay at each approach, weighted by the volume on each approach and represented by Eq. (11):

$$d\_{entire\\_roundabout} = \frac{\sum d\_{appreach,i}\, q\_i}{\sum q\_i} \tag{5}$$

where dentire roundabout is the control delay for the entire roundabout, s/veh; dapproach,k is the control delay for approach kth, s/veh; qi is the flow rate for approach i th, vph.

#### 4.3.2 Control delay at the entire intersection: comparing before and after solutions

It is imperative for a designer to understand the relationships between design features and crash frequency.

The effectiveness of all the changes that have been designed without changing the shape of a regular intersection but adjusting it to the Italian Standard (see Figure 8a and Section 4.3.1) was confirmed by the expected crash frequency values computed by adopting the SPF available in Biancardo et al. [16]. Biancardo et al. [16] worked in line with HSM2010 [17] procedure and revised the equation available in the Manual to predict crash frequency at three and four-leg rural unsignalized at-grade intersections.

The Nspf formulation [16] was here used (see Eq. (12)) to predict the crash frequency at the two-lane two-way four-leg intersections studied in greater depth (intersection D) as it was calibrated using a data set that adequately reflects and partly overlaps with what is explored here. MLW is the mean lean width of the approaching and departing lanes.

$$N\_{\rm pf} = AADT \cdot \exp\left[-1.042 \cdot MLW - 8.5\right] \tag{6}$$

Eq. (12) applies to an AADTmaj range from 0 to 14,700 vpd and AADTmin range from 0 to 3500 vpd.

In HSM2010 [17], Crash Modification Factors (CMFs) are introduced to account for the specific site conditions that differ from the hypothesized base conditions. Under base conditions, the CMF is 1.00 (i.e., Figure 5d), while the CMF is less than 1.00 when a geometric configuration in compliance with the Standard and with many additional modules exists and, consequently, a reduction of average yearly crash frequencies can be expected. Npredicted (predicted average crash frequency for a specific year for site type x) is shown in Eq. (13), where the effect of the skew angle does not appear, as study intersection D has an 80° angle, very close to orthogonal road axes and, consequently, no additional benefits can derive from further correction or the right-turn lanes that already exist on major roads.

b.Phase II: adding left-turn lanes on major-road approaches based on the ratio between the volume of vehicles turning left per hour and the total traffic

Advanced geometric design solutions for an existing non-circular intersection. (a) Adjustment according to the Italian Standard. (b) Changing the configuration into a compact roundabout (Dext = 26 m) according to the

Right-turn lanes on minor road stops are not permitted by the Italian Design

consequently new lighting and new right-turn lanes on major roads are not required. Control delay at the entire intersection for the first advanced geometric solution of intersection D (Figure 8a) was estimated as in Section 3.2; the expected LoS of this geometric adjustment is shown in Table 7. In particular, to compute the

17 C Intersection\* 9 A

\*Adjusted to the Italian Standard without changing the shape by adding further modules Figure 8a.

LoS before treatment

The intersections investigated are almost totally equipped with right-turn lanes on major roads and they have lighting within and approaching the intersection area;

Non-treatment site Expected configurations

Compact roundabout\*\*

Comparison of the control delays at advanced geometric solutions with those of intersection D (Figure 4, old

Site Control delay on the

entire site, s/veh

4 A

LoS after treatment

volume on the highway per hour.

Transportation Systems Analysis and Assessment

Control delay on the entire intersection, s/veh

\*\*Figure 8b.

configuration).

Table 7.

174

Figure 8.

Italian Standard.

Standard [14], and they are not included in the design.

$$N\_{predicted} = N\_{sf} \cdot \text{CMF}\_{LTL} \cdot \tag{7}$$

where Nspf was determined by the following Eq. (12).

CMFLTL was computed using the HSM procedure, and it benefits from the effects of the presence of left-turn lanes (LTL) on the major road, specifically in terms of expected average annual crash frequency reduction compared with what can be observed at intersections with a poor geometric configuration. CMFLTL is equal to 1 for four-leg unsignalized rural intersections that meet base conditions. It equals 0.13 for the left-turn lanes present [16].

Table 8 shows, in the light of the foregoing, the expected annual number of crashes if the intersection is adjusted to Italian Road Design Standard [14] requirements by introducing additional geometric modules as listed in the first part of Section 3.3.1.

Moving on now to the evaluation of the effectiveness of the second treatment (the conversion of typical intersections into compact roundabouts) suggested for intersection D in order to check whether the level of exposure to crash risk can be reduced and is generally well managed, the EB procedure was adopted, as mentioned in the Literature review section.

First of all, it is necessary to calculate the expected annual number of crashes (m) if conversion to a roundabout does not take place. Eq. (14) was adopted to obtain a site-specific estimate of the m variable at a typical intersection before conversion to a roundabout:

$$m = w\_1 \mathfrak{x} + w\_2 P \tag{8}$$

Rodegerdts et al. [18] suggested k equals 0.77 for an SPF that predicts the total number of crashes per year, and k equals 1.25 for an SPF that predicts the total number of injury crashes per year. In chapter C, [18] are defined the results of the efforts to develop intersection and approach-level models. These models relate crash prediction to the number of lanes, number of legs, and the average annual daily traffic. SPFs used to predict the expected total number of crashes per year at intersection (Eq. (17)) or the expected total crash injuries per year at intersection

Case study—Intersection D Crashes per year

Analytical Assessment of Effective Maintenance Operations on At-Grade Unsignalized…

P 0.89 0.20 K 0.77 1.25 w1 0.15 0.11 w2 0.23 0.45 m 2.37 0.64

Expected annual number of crashes 1.86 0.26 Expected changes in no. of crashes �0.51 �0.38 Reduction in no. of crashes �21% �59%

Calculation of the expected change in the number of crashes when intersection D is converted into a

BEFORE (neither adjustment nor conversion) (Eq. (14))

DOI: http://dx.doi.org/10.5772/intechopen.86435

AFTER (after conversion to roundabout—solution 2)

Eqs. (17) and (18) have been used in this study to predict m variable, since they

Eq. (19) was adopted to predict the expected total crash frequency per year

A comparison of the expected crash frequency between conversion and nonconversion into a single-lane roundabout of the four-leg two-way-stop intersection is performed by plotting Figure 9. This makes it possible to identify a maximum threshold for the AADTtotal entering at the single-lane roundabout when this configuration replaces an existing typical intersection without damaging the required

were validated using the data set that is adopted here as shown in [16].

equal to 7642 vpd for the intersection in question.

The expected safety effects are shown in Table 9.

after converting the intersection into a single-lane roundabout [19], where AADTtotal entering is the total annual average daily traffic entering the roundabout,

<sup>P</sup> <sup>¼</sup> exp ð Þ� �8:<sup>63</sup> AADTtotal entering <sup>0</sup>:<sup>952</sup> (11)

Total Injury

<sup>P</sup> <sup>¼</sup> exp ð Þ� �8:<sup>733</sup> AADTtotal entering <sup>0</sup>:<sup>795</sup> (12)

<sup>m</sup> <sup>¼</sup> <sup>0</sup>:<sup>023</sup> � AADTtotal entering <sup>0</sup>:<sup>749</sup> (13)

(Eq. (18)) are as follows:

Table 9.

roundabout.

5. Results and discussions

safety levels.

177

where m is the expected site-specific annual number of crashes or injury crashes before conversion; x is the count of crashes in the n years before conversion (see Table 2, a total of 14 crashes occurred in 5 years with 4 injuries); n = 5 is the study period in this research; w1 and w2 are weights, Eqs. (15) and (16) [18]:

$$w\_1 = \frac{P}{\frac{1}{k} + nP} \tag{9}$$

$$w\_2 = \frac{\frac{1}{K}}{\frac{1}{k} + nP} \tag{10}$$

where P is the prediction of the annual number of crashes, or the annual number of injury crashes depending on what it is necessary to investigate using an SPF to identify intersections with similar characteristics before conversion; k is the dispersion parameter for a given model, estimated from the SPF calibration process using a maximum likelihood procedure.


#### Table 8.

Calculation of the expected change to the number of crashes after shape adjustment in line with the requirements of the Italian Road Design Standard.

Analytical Assessment of Effective Maintenance Operations on At-Grade Unsignalized… DOI: http://dx.doi.org/10.5772/intechopen.86435


#### Table 9.

Npredicted ¼ Nspf � CMFLTL� (7)

m ¼ w1x þ w2P (8)

<sup>k</sup> <sup>þ</sup> nP (9)

<sup>k</sup> <sup>þ</sup> nP (10)

where Nspf was determined by the following Eq. (12).

equals 0.13 for the left-turn lanes present [16].

Transportation Systems Analysis and Assessment

mentioned in the Literature review section.

conversion to a roundabout:

a maximum likelihood procedure.

requirements of the Italian Road Design Standard.

Table 8.

176

Section 3.3.1.

CMFLTL was computed using the HSM procedure, and it benefits from the effects of the presence of left-turn lanes (LTL) on the major road, specifically in terms of expected average annual crash frequency reduction compared with what can be observed at intersections with a poor geometric configuration. CMFLTL is equal to 1 for four-leg unsignalized rural intersections that meet base conditions. It

Table 8 shows, in the light of the foregoing, the expected annual number of crashes if the intersection is adjusted to Italian Road Design Standard [14] requirements by introducing additional geometric modules as listed in the first part of

Moving on now to the evaluation of the effectiveness of the second treatment (the conversion of typical intersections into compact roundabouts) suggested for intersection D in order to check whether the level of exposure to crash risk can be

First of all, it is necessary to calculate the expected annual number of crashes (m) if conversion to a roundabout does not take place. Eq. (14) was adopted to obtain a site-specific estimate of the m variable at a typical intersection before

where m is the expected site-specific annual number of crashes or injury crashes before conversion; x is the count of crashes in the n years before conversion (see Table 2, a total of 14 crashes occurred in 5 years with 4 injuries); n = 5 is the study

> <sup>w</sup><sup>1</sup> <sup>¼</sup> <sup>P</sup> 1

> > 1 K 1

where P is the prediction of the annual number of crashes, or the annual number of injury crashes depending on what it is necessary to investigate using an SPF to identify intersections with similar characteristics before conversion; k is the dispersion parameter for a given model, estimated from the SPF calibration process using

w<sup>2</sup> ¼

Case study Crashes per year Intersection D Total Injury Expected annual number of crashes 1.99 0.57 Expected annual changes to the number of crashes �0.81 �0.23 Reduction in crashes �28% �29%

Calculation of the expected change to the number of crashes after shape adjustment in line with the

reduced and is generally well managed, the EB procedure was adopted, as

period in this research; w1 and w2 are weights, Eqs. (15) and (16) [18]:

Calculation of the expected change in the number of crashes when intersection D is converted into a roundabout.

Rodegerdts et al. [18] suggested k equals 0.77 for an SPF that predicts the total number of crashes per year, and k equals 1.25 for an SPF that predicts the total number of injury crashes per year. In chapter C, [18] are defined the results of the efforts to develop intersection and approach-level models. These models relate crash prediction to the number of lanes, number of legs, and the average annual daily traffic. SPFs used to predict the expected total number of crashes per year at intersection (Eq. (17)) or the expected total crash injuries per year at intersection (Eq. (18)) are as follows:

$$P = \exp\left(-8.63\right) \cdot \left(AADT\_{\text{total entering}}\right)^{0.952} \tag{11}$$

$$P = \exp\left(-8.733\right) \cdot \left(\text{AADT}\_{\text{total entering}}\right)^{0.795} \tag{12}$$

Eqs. (17) and (18) have been used in this study to predict m variable, since they were validated using the data set that is adopted here as shown in [16].

Eq. (19) was adopted to predict the expected total crash frequency per year after converting the intersection into a single-lane roundabout [19], where AADTtotal entering is the total annual average daily traffic entering the roundabout, equal to 7642 vpd for the intersection in question.

$$m = 0.023 \cdot \left( AADT\_{\text{total entering}} \right)^{0.749} \tag{13}$$

The expected safety effects are shown in Table 9.

## 5. Results and discussions

A comparison of the expected crash frequency between conversion and nonconversion into a single-lane roundabout of the four-leg two-way-stop intersection is performed by plotting Figure 9. This makes it possible to identify a maximum threshold for the AADTtotal entering at the single-lane roundabout when this configuration replaces an existing typical intersection without damaging the required safety levels.

road system on the one hand, and to analyze the effectiveness of treatment for the effective management of hotspots and ensure the good operation of the system, on the other hand. The procedure investigated can help in the allocation of resources according to the needs and severity of a possible crash event that, although rare, can have dramatic consequences, especially when risk factors are not identified, ana-

Analytical Assessment of Effective Maintenance Operations on At-Grade Unsignalized…

In this chapter, a methodological process that can also be implemented in other domains was shown to calculate, manage, and reduce, through appropriate treatments, the expected crash risk level measured in terms of yearly crash frequency

First of all, the procedure aimed to identify, and then manage, the hotspots on a rural road intersection network where high exposure to crash risk can be observed. It also sought to rank the hazardous sites, for which two measures of exposure to risk were suggested and assessed in line with the research presented, namely the crash rate and Level of Service in terms of control delay at the intersection area. Of course, a safe system approach requires a fundamental cultural and ethical shift in thinking, but it is also true that the current road transport system is not as safe as it could be. However (a) if the system could be well supervised, (b) if the trend of a number of system status indicators (i.e., crash rate level, level of service, crash cost, etc.) could be carefully plotted to check their decay over time, (c) if design errors were promptly identified, and (d) if the correlations between design errors/access management and factors that cause increased exposure to crash risk were then investigated, in the event of human error or driver distraction, the resulting severity might not be as high. Obviously, system designers and system users must all share responsibility for managing crash forces to a level that does not

It has been verified whether improvements can be achieved in terms of safety level (reduction of the number of crashes and injuries) and the quality of traffic (reduced control delay over the entire intersection) when the geometric design of existing intersections belonging to two-lane rural roads and located on a flat area

The experimental method covered two parallel trajectories that ultimately converge:

• adapting an existing at-grade intersection without changing its shape;

features and environmental conditions constant.

No potential conflict of interest was reported by the authors.

• changing its geometry according to the Italian standards, keeping traffic

The results show that for the intersections in question, designing a single-lane roundabout according to the Italian Road Design Standard, or an intersection introducing left-turn lanes, deceleration lanes, and median-refuge islands could help to achieve this goal. Compact roundabouts are, in any case, the best solution in terms of Level of Service and safety level because they contribute to strongly reducing

lyzed, and reduced.

DOI: http://dx.doi.org/10.5772/intechopen.86435

6. Conclusions

and Level of Service.

result in death or serious injury.

does not meet the Italian Standard.

delay as well as crashes.

Conflict of interest

179

Figure 9. Roundabout performance in terms of expected crash frequency for AADTtotal\_entering.

The results summarized in Table 10 and listed below highlight a strong correlation between the LoS and the safety level for managing hotspots along road networks and the corresponding crash risk levels, improving system quality for users. The results achieved show that, by increasing control delay throughout the entire intersection, the expected safety level for the expected annual number of crashes decreases. Conversely, when the estimated level of service increases (reducing the control delay of the entire intersection), the safety level improves (translating into a low value for the expected value of annual crashes per year). Results confirm that if a study intersection, under specific traffic conditions, and in a specific environmental and surrounding context, has a suitable and correct geometric configuration for reducing the number of conflict points during possible maneuvers, the control delay on the entire structure is reduced, and the LoS improves. This reflects indirectly, but positively, on the safety level because the expected value of the annual crashes decreases.

This research aimed to identify road strategies to improve road safety conditions at rural two-lane two-way intersections with stop-control in order to identify crash risk factors that may affect the Level of Service (LoS) and the safety level of the


#### Table 10.

LoS and expected crash frequency at advanced geometric solutions.

Analytical Assessment of Effective Maintenance Operations on At-Grade Unsignalized… DOI: http://dx.doi.org/10.5772/intechopen.86435

road system on the one hand, and to analyze the effectiveness of treatment for the effective management of hotspots and ensure the good operation of the system, on the other hand. The procedure investigated can help in the allocation of resources according to the needs and severity of a possible crash event that, although rare, can have dramatic consequences, especially when risk factors are not identified, analyzed, and reduced.
