Physical and economic impacts of studded tyre use on pavement structures in cold climates

ABSTRACT

1. Introduction

In cold regions, studded tyre use is considered a factor that contributes to pavement rutting and damage. In Alaska of USA, just like in other cold environments, pavement deterioration leads to an increased cost associated with pavement resurfacing [1]. The heavy wheel loads of trucks also cause noteworthy damage to highway pavements. The Alaska Department of Transportation and Public Facilities (DOT&PF) is concerned about this issue and seeks feasible solutions to mitigate the damage, which is the main purpose of this paper.

In many ways, Alaska is a unique state. It is well known for extreme temperatures, particularly in Interior Alaska, where winter temperatures have been recorded as low as –40 °F(–40 °C), and summer temperatures have been recorded as high as 100 °F(37.8 °C). This extreme temperature range makes it challenging to construct roadways and transportation facilities [2]. Due to its northern latitude, some locations in Alaska experience 24 hours of daylight in summer and nearly total darkness in winter, adding further challenges. Alaska's immense size, coupled with high mountain ranges and huge glacier fields, makes the cost of building roads prohibitive in much of the state, particularly in villages and towns in rural Alaska [3].

One common pavement defect caused by excessive use of studded tyres is ‘rutting’. The leading countries in studded tyre use are Nordic countries, especially Finland and Sweden. Studded tyre use estimates range from 95% in Finland to 49% in Alaska [4, 5]. In Alaska, historical studded tyre use was 73% in 1970 and decreased to 49% in 1990 [5]. The percentage has remained about the same from 1990 to 2003 [6]. Sweden has mandated winter tyre use and lately asserted use during winter months. A 1996 study in Oregon estimated the annual cost to repair damage caused to its highways by studded tyres, prior to lightweight stud regulations, at $37 million (1994 USD); the study reported asphalt pavements experiencing average daily traffic (ADT) volumes of 35 000 and 20% studded-tyre use will reach the threshold rut depth of 19 mm (0.75 in.) in 7 years, while Portland cement concrete pavements experiencing 120 000 ADT and 20% studded-tyre use will reach the threshold rut depth in 8 years [7].

Another study in Oregon quantified the current use of studded tyres, wear rates and associated costs. Several techniques were used to account for the extent of studded tyre use, from car park surveys to household surveys. That study showed a decline in studded tyre use from 16% in 1995 to 4% in 2013 for registered vehicles during winter months; the study also calculated an asphalt pavement wear rate of 0.0 295 in. (0.0 749 mm) per 100 000 studded tyre passes [8]. The present study used a comparable methodology to assess studded tyre use on roads in Alaska and to quantify the net damage cost of pavement surfaces.

2. Literature review

The review of literature in this paper was carried out to identify the previously reported studded-tyre impacts on pavement structures across different states. In addition, to identify the commercially available winter tyre options in Alaska as well as the current use of studded tyres and the cost caused by that use.

2.1 History of studded tyres

In the early 1960s,  studded tyres were introduced in the USA and became popular in cold regions. The built-in traction of studded tyres increased drivers’ self-confidence and eliminated the problems associated with installing temporary aids, such as tyre chains. Although studded tyres are generally accepted by the public as a means of enhancing mobility and safety, they have long been the source of considerable controversy. In many states, studded tyre use approached 30% of passenger vehicles by 1972, and in Alaska, Montana and Vermont, approximately 60% of passenger vehicles were equipped with studded tyres [9]. Approximately 10% of passenger vehicles in western Washington use two or more studded tyres, and approximately 32% of passenger vehicles in eastern Washington use two or more studded tyres [10].

Unfortunately, the studs have caused substantial pavement damage, which resulted in high maintenance costs for the road holders. In 1971, studded tyres were banned in a few states in the USA and Canada [4]. A Norwegian road grip study in 1997 led to an attempt to decrease studded tyre use in Norway's largest cities [4]. Likewise, Germany, Poland, the Czech Republic, and Japan have banned studded tyre use. In Japan, tyres with metal studs were banned in part because of the health hazards created along highways during winter months from damaged concrete [5].

2.2 Types of winter tyres

In the United States, there are two major types of winter tyres: studded or non-studded. Non-studded winter tyres are composed of advanced rubber compounds or additives to increase traction. In general, non-studded tyres designed for passenger vehicles are constructed with soft rubber compounds. Trucks and heavier vehicles use non-studded tyres that are made of hard rubber compounds that last longer under the heavy axle weight.

Scheibe [10] compiled performance-based data from a number of sources and provided conclusions about winter-driving traction aids. The traction of studded tyres is slightly superior to non-studded tyres only under a specific circumstances, clear ice near the freezing point, a condition with limited occurrence. For most test results reviewed for snow and for ice at lower temperatures, studded tyres performed as well as or worse than non-studded tyres. For those conditions in which studded tyres provided better traction than non-studded tyres, the difference was usually small. The precise environmental conditions under which studded tyres provide a traction benefit are relatively rare. However, the relative frictional gain of studded tyres diminishes or becomes negative on roughened ice, as temperature drops and as the studs wear.

2.3 Impact of studded tyres on pavement structures

Studded tyres contribute to the rutting of roadways in Alaska and contribute partially to the wear of marking stripes. In Alaska's central and south coast regions, the lifespan of pavement subject to studded tyre wear is unspecified, but is far shorter than the lifespan of pavement in the Lower 48. Based on past construction projects in Anchorage, pavement resurfacing life due to rutting ranges from 7 to 9 years with an average of 8 years for freeways, and higher for other road classes, such as arterials and collectors. Previous studies showed that studded-tyre use, regardless of its other benefits, inflicts damage on road systems. Studded tyres contribute to the wear of HMA (hot-mix asphalt) and concrete pavement, eventually forming ruts on the pavement surface. This is why some states set seasonal restrictions or prohibit studded tyres entirely.

Studded tyre wear is considered one of the major distressers affecting roadways in Alaska, especially on higher-volume roads in the central region. The early rut monitoring programmes that were carried out by Alaska DOT&PF reported that the studded-tyre wear rate in winter was significantly more than the rut caused by plastic deformation in summer [9], as shown in Fig. 1.

Fig. 1.

Rut depth progression [9]

Open in new tabDownload slide

2.4 Studded tyres wear rates

The scientific literature review showed that different pavement wear rates were published earlier in other states like Alaska, Washington and Oregon. The Alaska DOT&PF estimated wear rates of 0.102–0.148 in. (0.259-0.37 592 mm) per million passes in 1996 [11]. Oregon's DOT reported wear rates of 0.34 in. (0.864 mm) per million passes [12]. In addition, a technical brief from the Washington Department of Transportation reported total annual pavement damages of $8–$10 million on Oregon highways and $16 million annual pavement damages on Washington highways [13].

Shippen et al. [14] quantified the current use of studded tyres and the wear and cost caused by that use in Oregon. The study reported a decline in studded-tyre use from about 16% of registered vehicles in 1995 to about 4% in the 2013–2014 winter seasons. The wear rate of Portland cement concrete (PCC) was about 0.0 091 in. (0.0 231 mm) per 100 000 studded tyre passes, while the wear rate of asphalt pavement was about 0.0 295 in. (0.0 749 mm) per 100 000 studded tyre passes. Similarly, a published technical brief [15] discussed the total Washington statewide asphalt cost due to studded tyres. The rutting due to studs depends on the wear rate and the number of vehicles with studded tyres. The report concluded that studded tyre use rates vary from the west side of Washington (about 9% of vehicles) to the east side of Washington (about 25% of vehicles).

Gray [16] qualitatively supported the premise that there is no social or safety benefit from studded-tyre use in Oregon. Quantitative cost analysis was limited to pavement rutting on the state highway system that is sufficient to reduce the useful life cycle of the pavement. A range of wear rates was estimated, reflecting the numerous factors that influence rutting susceptibility of pavements. The mid-points of wear rates for asphalt and PCC were 0.0 386 in. (0.0 980 mm) and 0.0 093 in. (0.0 236 mm), respectively. Brunette and Lundy [7] reported that studded-tyre wear shortens pavement life on high-volume routes in Oregon. Asphalt pavements that experience ADT volumes of 35 000 and 20% studded-tyre use were found to reach the threshold rut (3/4 in. (19 mm)) in 7 years. PCC pavements that experience 120 000 ADT and 20% studded-tyre use were found to develop the threshold rut depth of 3/4 in. (19 mm) in 8 years.

According to a study done in Sweden by Jacobson and Hornvall [17], wear was measured through the SPS ratio (specific wear, grams of abraded material per vehicle with studded tyres, and kilometre). This measure has no constant for a certain pavement type, but an approximate estimate of actual wear in specific conditions and during a specific period. The SPS average has decreased from 30 during the late 1980s to 8 at the turn of the century. The most wear-resistant pavements have SPS ratios of 2–4. In the winter season of 1994-1995, wear was calculated to be 300 000 tons; in the late 1990s,  wear had diminished to around 110 000 tons. In Minnesota, Preus [18] reported the average terminal wear rates for normal bituminous wearing courses ranged between 0.75 in. (19 mm) and 0.95 in. (24 mm) per million studded-tyre passes. For conventional concrete pavements, the corresponding wear rates ranged from 0.30 to 0.47 in. (7.6 to 1.2 mm) per million studded-tyre passes.

2.5 Contribution of studs to total rut depth

Rutting in HMA pavement is apparent in two main forms: either deformation from wheel loads on pavement that is insufficient to support heavy truck weight, or from tyre wear, especially studded tyre wear. Studded tyres dig into the pavement and pick out small aggregate, eventually forming ruts. Based on this literature review, no studies have shown any practical method to reduce load rut and stud rut, although the DOT&PF tried to use polymer in reducing rut and the initial results were positive. Both rut forms are quite distinct in cause and appearance.

The dual wheel width of a truck exceeds the width of a studded-tyre groove (or rut); the wheels of a passenger vehicle lay directly within the wear pattern. The dynamics of studded-tyre action include three phases: as the studded tyre moves over the pavement, there are ‘spikes’ in force at the beginning and at the end of the contact. During these spikes, energy is transferred to the pavement in the form of scratching. Between these spikes, the studs have a ‘punching’ action that breaks up aggregate and picks out the pavement surface.

Assuming that 100% of trucks are moving on the right lane, the total rut measurements on this lane are due to the axle wheel loads of heavy trucks because no studs are impeded in the truck tyres. Based on this assumption, 100% of rutting in the left lane is due to studded-tyre passes, excluding any rut measurements wider than the normal tyre of passenger vehicles [12].

2.6 Surveys and level of studded-tyre use

Malik [12] discussed the use of studded tyres in Oregon. According to Malik's research approach, the level of studded-tyre use in Oregon was determined using two methods: car park surveys and household telephone surveys. During the winter of 1994-1995, the Oregon DOT conducted a car park survey of studded tyre use in Oregon. Heavily used parking areas, mostly at shopping centres, were selected at various locations to represent Oregon DOT's five regions. At each parking location and at each time, data were collected from 200 parked cars, indicating if the vehicle had 2-wheel or 4-wheel drive, and if studded tyres were mounted on front, rear, or both axles. In most cases, six visits were made to each location. All visits took place between the last week of November and the end of March. No visits took place during April, although studded-tyre use was permitted during that month. The car park survey results indicate an average statewide level of studded-tyre use of 18.15%.

Copple [19] counted only vehicles with Michigan licence plates. After selecting a site, a cluster containing a predetermined number of vehicles was surveyed in order of physical location. Selected sites were primarily car parks, but in smaller towns, vehicles parked on streets were surveyed. As a result, in this survey, the percentage of passenger cars using studs was 19.5%, while the percentage of pickup and panel trucks was 18.3% and the percentage of 4-wheel drive vehicles was 4.5%.

Preus [18] reported that data collection was carried out in Minnesota between February and 1 May 1970 and from 15 October 1970 to 4 January 1971. About 84 000 questionnaires were mailed, with a return of 47%. The questionnaire served two main functions: to determine the proportion of vehicles equipped with each type of tyre and to measure the amount of tyre exposure to various road cover conditions. Responses from the questionnaire revealed the following for the total study period: 36% of autos were equipped with studded tyres, but only about 1% of autos had them on all four wheels. Thirty-eight percent of driving during the study period was with studded tyres, about 23% of driving was with snow tyres and about 39% of driving was with all-season tyres.

Brunette and Lundy [7] undertook a relatively small data sampling and augmented some of the new Oregon DOT car park data to develop estimates of the level of studded-tyre use. A car park survey and an extensive telephone survey were conducted. According to the car park data, approximately half of all vehicles using studded tyres had them on both axles, effectively doubling the studded-tyre passes for those vehicles. The researchers estimate the statewide average use of studded tyres at 23.8%, with regional rates ranging from 65.7% to 7.4%.

3. Methodology

To estimate the current percentage of studded-tyre use in Alaska, a car park survey and an online household survey were conducted by the Department of Civil Engineering at the University of Alaska Anchorage. Fig. 2 addresses the methodology used in this research. A total of 1 226 vehicles were surveyed in the car parks throughout the Anchorage area. More than 800 households, altogether owning 1 531 vehicles, responded to the household online survey. This was followed by selecting sites for rut depth measurements and traffic data from several samples including freeway, arterial and collector roads as a case study. Data were collected from the Pavement Management and Statewide Planning teams at the Alaska DOT&PF. Wear rate estimates from studded-tyre traffic and truck traffic were calculated for each freeway sample. After establishing the theme from freeways and determining the contribution of stud wear on pavement, a comparable methodology was applied for arterial and collector roads. Pavement repaving/resurfacing life cost was estimated from as-built of 20 real historical projects. Finally, loss of pavement life was estimated for different road classes.

Fig. 2.

Research methodology

Open in new tabDownload slide

3.2 Data collection

The research method was carried out by collecting different types of data sets that include estimates of studded-tyre use, traffic volumes, traffic classification, roadway characteristics and rut depth measurements. These data collection procedures are explained in the following subsections.

3.2.1 Percentage of studded-tyre use

As previously mentioned, a parking survey was conducted from a sample of car parks in the Anchorage area. The survey covered heavily used parking areas, mostly at shopping centres and major generators. Seven sites were selected in Anchorage across different sections of the city to gain a diversity of respondents. The sample population represented a variety of income levels and educational levels. An additional site in Eagle River was considered to cover a broader geographic region, although this was accomplished primarily through the online survey, which covered all regions of Alaska. The selected sites include public institutions, commercial centres, private institutions, and shopping centers.

where n is the sample size, Z is a number based on the confidence level, p and q are the variances of the population and E is the maximum error of the estimation.

The confidence level is 95% (Z = 1.96) and the margin of error is 5%. The most conservative variance estimates for both p and q are 0.5. The calculation of sample size determined that a minimum of 385 distinct vehicles were needed for the survey. The research team observed at least 75 vehicles at each of the eight sites, nearly doubling the minimum required for the purposes of this survey. The survey was conducted twice for each site to verify possible inconsistencies of the data collection between visits for the same site. General information of the parking site, including region within the city, type of business, parking type, parking system and payment method, etc. were recorded. Then the following information was obtained about a minimum of 75 vehicles per site: vehicle type, drive type, wheel type (studded or non-studded), use of studs (front, rear, or both).

In addition to the car park survey, an online Qualtrics survey was programmed and sent out to the public through different outlets. The survey was programmed not only to determine the percentage of studded-tyre use, but also to capture the public point of view on using studded tyres or any other alternatives. Different questions were designed to test public awareness and the public's experience with new technology in winter tyres. The survey responses were received from more than 800 households, owning 1 531 vehicles altogether. These households represent a balanced sample relative to the population from all of Alaska's major cities including Anchorage, Palmer, Wasilla, Fairbanks, Juneau and Kenai.

3.2.2 Traffic data and rut depth measurements

Alaska traffic data and rut depth measurements were derived from a sample of freeways, arterials and collector roads in the state. Statistically, a minimum sample size of three sites for each roadway classification was considered for the significant wear rate analysis. These sites were selected based on the available actual data before any resurfacing, maintenance or rehabilitation projects done by the Alaska DOT&PF. Data on annual average daily traffic (AADT) and highway traffic data were collected from permanent stations located on various highway segments. Other traffic characteristics, such as growth rates and average monthly daily traffic, were taken from Alaska DOT&PF Traffic Volume Reports, published annually on the department's website [20]. For each permanent counter location, data sets were tabulated and classified by a directional split to define the percentage of traffic moving in each direction, by a lane split to show the distribution of traffic among the right and left lanes, and by vehicle classification to indicate the percentage of passenger vehicle and truck traffic. In addition, traffic growth rates were used to estimate the total average daily traffic encountered over the total pavement rehabilitation life for each roadway segment.

3.3 Wear rates analysis

It is hard to identify the pavement damage from studded tyres caused in a specific year, as the life of a pavement spans many years and collected rut measurements are the cumulative fractions of inches that develop over time. The pavement design life is 15 years for all classes of roadways in the urban area of Anchorage based on the Alaska flexible pavement design manual [21]. An estimate was derived for cumulative studded-tyre wear. First, the total number of years was calculated for each highway segment from the last resurfacing project occurred on that segment until that segment's pavement reached the rut threshold, or until the segment's next resurfacing project date was scheduled by Alaska DOT&PF. Then, an estimate for the total number of studded-tyre passes was calculated for each highway segment based on the following criteria: adjusted total traffic volume data using factors for the relative level of traffic during the studded-tyre season, from 15 September until 1 May (regional differences apply here for projects outside Alaska's central region); the percentage of traffic made up of total passenger vehicles and trucks; the proportion of vehicles in overall traffic volume using studded tyres.

A ‘rut’ is expressed as a function of cumulative studded-tyre passes over the road surface to identify the wear rate general model under the following assumptions: (a) the wear rate is constant because it stabilizes after 100 000 studded-tyre passes [12], and (b) it was assumed in the initial step of the calculation that all rutting in the left lane of a typical roadway is caused by studded tyres resulting from passenger vehicles only. Then the rut rate is calculated based on the actual percentage of trucks on each lane.

After establishing the theme from freeways and determining the contribution of stud wear on the pavement, a comparable methodology was applied for the arterial and collector roads. Because many factors that affect wear rate were present, the data were analysed under the same conditions to eliminate the contribution of these factors. Variables, such as speed, pavement design and materials, were constant for each highway segment. The only variables taken into consideration were traffic volume and traffic classification on highway segments. The wear rate estimate is based on the assumption that the same type of metal studs, commonly used in the U.S. tyre market, are used.

Each data set in the rut measurements was combined with traffic data and current estimates of studded-tyre use. No information is available in the literature that shows methods to differentiate between rutting wear from studs and rutting wear from wheel loads. It was assumed that trucks travel predominantly in the right lane. A study done in Oregon [12] resolved this challenge by summing the rut depth of each lane for every highway segment, then performing a regression of the combined depth against total directional studded-tyre traffic.

According to Alaska traffic law in the state's central region, 7.5 months is the time allowed for the public to use studded tyres—from 15 September to 1 May. The AADT in the total number of days during that period was multiplied by the percentage of traffic split between the right and left lanes to get the total number of vehicles in the respective lane. In addition, studies of studded-tyre rutting have shown that pavement surfaces have a higher initial wear rate. Rut rates stabilize after 100 000 studded-tyre passes [12]. Therefore, wear rates were calculated per 100 000 entering vehicles and trucks for each highway segment. Wear rates were expressed as a function of rut depth over traffic volume. The wear rate models were generated for each highway sample, as shown in the equations below.

First, the left lane's wear rate, which was calculated considering ruts that result exclusively from passenger vehicles, was calculated using Equation (2),

where |$W{R_{PV\_left}}$| is the wear rate due to passenger vehicle in the left lane (in./100 000 passes), |$Traffi{c_{7.5month}}$| is the total amount of traffic during the winter season of the period considered, |$Ru{t_{left}}$| is the rut depth observed in the left lane (inches), |$Left\_lan{e_{split}}$| is the percentage of traffic moving in the left lane and |$\% Studs$| is the percentage of passenger vehicles using studded tyres.

where |$\% P{V_{left}}$| is the percentage of passenger vehicles moving in the left lane.

where |$W{R_{PV\_right}}$| is the wear rate estimate due to studded passenger vehicles on the right lane (in./100 000 passes), and |$\% P{V_{right}}$| is the percentage of passenger vehicles moving on the right lane.

4. Results and discussion

As a case study, the methodology was applied on several samples from Alaska's roadway networks including freeways, arterials and collectors. The route names and total miles considered for the analysis are shown in Table 1 (1 mile=1.6 km).

4.1 Car park and household survey results

A total of 1 226 vehicles were surveyed from car parks throughout the Anchorage area. Most of the surveyed vehicles were SUVs (45%), followed by passenger cars (32%), then commercial trucks (19%) and lastly vans (4%). Sixty one percent of the vehicles were non-studded while 35% were studded, the rest are all weather or non-studded winter tires. The percentage of vehicles with studded tyres, differentiated by location, are shown in Fig. 3. The figure indicates that the range of results between two visits is not significantly different, except in the following locations: providence hospital, DOT and Walmart. The state-owned vehicles at the DOT&PF aviation building might have biased the results because most state vehicles have studded tyres. Further discussion is provided in the next section.

On the first visit of the car park survey, the average studded-tyre use was 34% with a standard deviation of 4%, whereas the average was 36% with a standard deviation of 6% for the second visit. Overall, the average studded-tyre use was 35% with a standard deviation of 5%. On the other hand, the household survey responses showed an average studded-tyre use of 48.6%. A notable result from this survey is that 63.0% of the sample is considering switching from studded tyres to winter tyres with new technology, a trend that might decrease the percentage of studded-tyre traffic in the future. Also, 37.0% of the sample is not considering non-studded winter tyres because of safety concerns (54.6%), cost concerns (14.6%) and for a variety of other reasons, including a lack of awareness about non-studded alternatives.

Some households responded that they are aware of the performance of non-studded winter tyres, but considered that studded tyres are essential for winter driving, especially in hilly or mountainous areas to improve overall driving performance and safety, neglecting the fact that studs can cause rapid deterioration of pavement, which can lead to other safety hazards. Most responses came from people at 31–40 years of age (223 responses), 51–60 years of age (213 responses) and 21–30 years of age (201 responses). The fewest responses came from people at 18–20 years of age (71 responses). Of the responses, 731 households own a single vehicle; 455 of those vehicles are all-wheel drive, and 372 have studded tyres on all wheels. Also, 585 households own a second vehicle; 386 of those vehicles are all-wheel drive, and 242 have studded tyres on all wheels. Based on the respondents’ experience with using new technology in winter tyres, 62.6% are not considering switching back to studs. Most respondents do not realize that non-studded winter tyres, in fact, provide traction and safety performance that is comparable to studded tyres.

Results of the household online survey and parking survey reported differences in the percentage of studded-tyre use. The online survey covered all regions of Alaska and the parking survey covered only the Anchorage area. The Oregon study on which the methodology was based compared the findings from a telephone survey and car park surveys. The studded-tyre usage from the car park and telephone surveys for Region 5 of Oregon State was inconsistent, with about a 20% difference, although results for other regions were reasonably consistent. Finally, the study reported the overall studded-tyre usage from telephone surveys, which is the lower of the two values.

4.2 Wear rate results

The freeway samples showed significant wear rates because of studded tyres on the right lane that were higher than the wear caused by wheel loads. Figs. 4 and 5 show a sample distribution of rut depth measurements and wear rates on Glenn Highway as an example (1 inch= 25.4 mm).

Fig. 3.

Studded tyre use in different car parks

Open in new tabDownload slide

Fig. 4.

Glenn Highway rut depth

Open in new tabDownload slide

Fig. 5.

Glenn Highway wear rates

Open in new tabDownload slide

Results from the freeway segments showed significantly higher average wear rates due to studded passenger vehicles—wear rates that reach 0.0 116  in.(0.2 946 mm)/100 000 studded vehicles compared with average rut rates on the right lane due to heavy wheel loads that reach 0.0 049  in.(0.1 244 mm)/100 000 trucks. These results support the claim that studded tyres contribute to pavement deterioration more than heavy wheel loads.

In the case of arterial and collector roads, it was more difficult to differentiate between rutting caused by wheel loads and rutting caused by studded-tyre traffic. A comparable methodology was applied over arterial and collector roads by assuming the same truck rut rates from freeways. An average truck rut rate of 0.0 049  in.(0.1 244 mm)/100 000 trucks was assumed for arterial samples. Then, rut measurements due to the percentage of truck traffic were calculated for each arterial segment. Rut measurements because of passenger vehicles were estimated by subtracting the rutting caused by truck traffic from the total rut depth. Finally, wear rates due to passenger vehicles were generated for each arterial segment. The results show a significantly lower average wear rate due to studded passenger vehicles on arterial roads, reaching 0.0 062  in.(0.1 575 mm)/100 000 studded vehicles, compared with an average wear rate of 0.0 116  in.(0.2 946 mm)/100 000 on freeway segments. Moreover, the results show a significantly lower average wear rate due to studded passenger vehicles on collector roads, reaching 0.0 045  in.(0.1 143 mm)/100 000 studded vehicles.

4.3 Cost estimates

The cost estimate is divided into three subsections, one for each type of cost analysis that was employed. These estimates include pavement resurfacing and rehabilitation costs, pavement damage costs due to studded tyres, and costs due to the reduction of pavement life caused by studded tyres. Cost estimates were generated using wear rates and studded-tyre traffic data for the selected freeway segments in the case study. All cost estimates were expressed in terms of resurfacing and rehabilitation costs.

4.3.1 Resurfacing and rehabilitation cost estimates

Several asphalt mix designs with different structural sections were considered to calculate the unit cost per square foot for resurfacing and rehabilitation. Data for these structural sections and total price per ton are shown in Table 2. The repair costs were limited to a rehabilitation strategy of the structural section thickness (mill/fill).

A list of 20 actual projects was developed, as shown in Table 3. Data from these projects were extracted from real as-built drawings to reflect the actual quantities used during construction. First, a realistic cost estimate was determined per pavement square foot of construction, which includes overall resurfacing costs (milling, striping, traffic maintenance and control). Then, the cost of total pavement damage was estimated for the rutting damage, including rutting damage that reaches the rut threshold limit. Based on feedback from Alaska DOT&PF, a 0.5 in. rut threshold limit was taken into consideration to determine the cost of pavement resurfacing and rehabilitation. However, the exact weighted average for the rut threshold was estimated from the highway samples, including freeways, arterials and collectors, to capture a range of costs and to provide future prediction cost estimates for Alaska DOT&PF. Finally, the pavement resurfacing cost was calculated for the 20 projects to establish a realistically estimated cost of construction/rehabilitation. Table 3 shows the calculated cost per square foot (1 ft² = 0.09 m²) for each project.

4.3.2 Pavement damage cost estimates due to studded tyres

The total number of lane miles is equivalent to the total rut depth reaching the threshold of 0.5 in.(12.7 mm). The resurfacing cost per square foot (0.09 m), as noted in Table 3, was multiplied by 63 360 ft²(5 818 m²) to convert 1 ft² to get the total cost per one lane-mile (1 ft |$\times$| 12 ft (0.06 m |$\times$| 3.3 m)) for the freeway samples shown in Table 4.

4.3.3 Cost estimates due to a reduction in pavement life

For example, for the Glenn Highway, which is a freeway selected for the case study, with an AADT of 10 000 vehicles per lane and 35% of vehicles having studded tyres, there are 3 500 vehicles with studded tyres using that road per day. From 15 September to 1 May or for 227 days, there are 794 500 vehicles with studded tyres per year on that segment of the highway, or 794 500 studded passes per year. Using the wear rate of 0.0 116 in.(0.2 946 mm) per 100 000 studded-tyre passes, this level of traffic equates to 0.0 922 in.(2.342 mm) of studded-tyre wear per year. For a rut threshold of 0.5 in.(12.7 mm), this roadway segment would need to be rehabilitated after 5.42 years. The normal pavement resurfacing cycle based on different threshold rut values of typical freeways in Anchorage ranges from 7 to 9 years, with an average of 8 years. Because the pavement design life in Anchorage is 15 years [21], the effect of studded tyres reduces the asphalt surface life by 6–8 years with an average of 7 years, which is about 47% loss of pavement life. Similarly, the reduction in pavement life was found for arterials and collectors to be 5 years and 3 years, respectively. With a given asphalt paving budget for Alaska statewide and with the percentage in the reduction of pavement life, the total asphalt cost due to studded tyres can be estimated as a monetary value.

4.3.4 Statewide cost estimate and countermeasures

All paved roads statewide, excluding unpaved or gravel roads, were analysed in this paper for resurfacing and rehabilitation needs. The cost of pavement rehabilitation from Table 3 was used as an estimate of the total resurfacing cost of mitigation. The estimate of the cost took into consideration studded-tyre use, growth in traffic, studded-tyre season length, the adoption rate of non-studded tyres, proportion of heavy load vehicles, average rut rate due to studded passenger vehicles, and rut rate due to heavy wheel loads. Although the detailed statewide economic analysis and cost estimates can be found in the study done by Abaza et al. [1], a summary of the results are described here to support the research hypothesis.

The results from this analysis show that the estimated total cost of mitigating road damage from studded tyres in Alaska over the next 20 years is $203.2 million in 2019 USD, discounting any future damages by 3%. The effective annualized damage cost associated with studded tyres still amounts to $13.7 million annually. The Alaska department of revenue analysed tyre fees for the past 6 years. Published annual fees from studded-tyre sales and stud installations were divided by the tyre fee of $5 to calculate the number of studded tyres and stud installations sold each year [22]. Comparing the effective annualized damage to the annualized studded-tyre fees of $318 000, the resurfacing cost associated with road damage from studded tyre use is more than 42 times larger than the state's fees from the sale of studded tyres and stud installations.

According to the analysis shown above, this paper proposed a set of countermeasures to mitigate the studded tyre damage in Alaska. These countermeasures were proven effective based on the analysis by Abaza et al. [1]. The first countermeasure is to phase out the allowed use of studded tyres. This would result in the elimination of current statewide annualized damages of $13.7 million and eliminate damages of almost $203 million over the next 20 years without additional cost to the state and consumers, as nonstudded-tyre options are similar in cost and safety to studded tyres.

The second countermeasure is to ban the use of heavy metal studs and switch to lightweight studs. This would result in a net cost savings of 50% in total pavement damage. Based on previous research results [1], the total pavement rehabilitation life of the Alaska roadway system would increase by 7% to 10%.

The third countermeasure is to shorten the studded-tyre season by 2 weeks on both ends, consistent with recently observed climatic changes. This would allow studded-tyre use between 1 October and 15 April, which would shorten the current season by 4 weeks and reduce annualized studded-tyre damage by $3.2 million, leaving $10.5 million in annualized damages.

The fourth countermeasure is to educate motorists about the safety of non-studded winter tyres. A 9% reduction in studded-tyre use and encouraging motorists to switch to non-studded winter tyres every year would result in a decrease of the annualized damage by $4.5 million, still leaving $9.2 million in annualized damages.

5. Conclusions and recommendations

In order to quantify the degree of pavement damage caused by studded-tyre use in Alaska, rut measurements and traffic data were collected from a sample of the state's freeways, arterials and collectors. Data were classified per directional split, lane split and vehicle classifications including passenger vehicles and heavy trucks. Car park surveys and online household surveys were employed to determine an approximate value of studded-tyre use. A total of 1 226 vehicles were surveyed in car parks throughout the Anchorage area where studded-tyre use was found to be 35%. More than 800 households, altogether owning 1 531 vehicles, responded to the household online survey. Data were analysed and tabulated to differentiate between rutting caused by passenger vehicles using studded tyres (wear) and rutting caused by trucks with heavy wheel axial loads (plastic deformation).

The following conclusions can be drawn from this research.

1. Freeway segments show significantly higher average wear rates due to studded passenger vehicles, 0.0 116 in.(0.2 946 mm) per 100 000 studded vehicles, compared with average rut rates due to heavy wheel loads on the right lane, 0.0 049 in.(0.1 244 mm) per 100 000 trucks.

2. There are significantly lower average wear rates on arterial and collector roads due to studded passenger vehicles, reaching 0.0 062 in. and 0.0 045 in.(0.1 143 mm) per 100 000 studded vehicles, respectively.

3. The average cost of pavement resurfacing ranged from $2.06 to $3.24 per square foot, based on the pavement structural section.

4. Estimates show that studded-tyre use reduced asphalt surface life on the selected freeway sample by about 7 years, which is about 47% loss in pavement life based on the initial design life of 15 years; other classes of roads experienced a lower reduction in service life.

5. The estimated total cost of mitigating road damage from studded tyres in Alaska over the next 20 years is $203.2 million in 2019 USD.

6. Comparing the effective annualized damage to the annualized studded-tyre fees of $318 000, the resurfacing cost associated with road damage from studded tyre use is more than 42 times larger than the state's fees from the sale of studded tyres and stud installations.

Base on the outcome of this research a set of countermeasures were proposed to help DOTs and transportation agencies to reduce the impacts of studded tyres. The output of this study is recommended to be considered by planners and highway engineers when deciding to invest in state roadway maintenance and rehabilitation projects in cold regions. It is recommended to evaluate the use of studded-tyre use with time to evaluate the shift into the use of winter nonstudded-tyre technology. In addition, evaluate the geometry of the rut in the determination of the cause of the rut, passenger vehicles versus commercial trucks. Furthermore, evaluate the safety impacts of rut geometry and depth in terms of accidents.

ACKNOWLEDGEMENTS

The authors of this paper are grateful for the financial support and the sponsor of this research from the Alaska Department of Transportation and Public Facilities (DOT&PF) with funds from the FHWA under project number FHWA-AK-RD-4000(132), Z630680000. The original report can be found at Alaska DOT&PF.

Data availability statement

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Conflict of interest statement. None declared.

REFERENCES