The Roadside Soil as a Chemical Archive of Urban Traffic

A roadside strip of soil can appear unremarkable: compacted earth, a few weeds, dust, litter and the occasional puddle. Yet this narrow boundary between a road and the surrounding landscape receives a continuous mixture of materials generated by modern transport. Fragments of tyres, brake-wear particles, road-surface debris, trace metals, fuel-related residues and chemicals released from vehicle components can accumulate there over years or decades.

Unlike an exhaust plume, which disperses through the atmosphere, many traffic-related contaminants can remain associated with soil particles. Some bind to organic matter or mineral surfaces; others move with runoff, infiltrate deeper layers or are carried into drains, rivers and agricultural land. Their concentrations and chemical forms change as the environment weathers, oxidises, dilutes or redistributes them.

This makes roadside soil more than a pollution sink. Under suitable conditions, it can function as a chemical archive of urban transport: a record of changing traffic intensity, vehicle technology, industrial activity, road maintenance and environmental management.

The archive is imperfect. Soil can preserve some historical signals while losing others. Construction, resurfacing, excavation, erosion and the addition of new sediment may mix or erase earlier layers. Furthermore, the same contaminant may originate from multiple sources. Lead, zinc or copper detected beside a road cannot automatically be attributed to traffic alone.

Nevertheless, combining chemical measurements with spatial patterns, soil depth, historical records and source-specific indicators can reveal how traffic pollution accumulates and moves through urban environments.

The importance of this perspective is growing. As cities expand, vehicle fleets change and tyre-derived pollutants receive greater scientific attention, environmental assessment must look beyond tailpipe emissions. It must also examine the material residues that transport systems leave behind.

For India, the question is particularly relevant. Dense traffic, diverse vehicle fleets, rapid urbanisation, industrial activity, exposed roadside soils and intense seasonal rainfall create a complex setting in which contaminants may accumulate in one location and be redistributed to another. Understanding this process requires more than measuring total metal concentrations. It requires reconstructing the pathways through which pollutants enter, persist in and leave roadside environments.

Why soil can preserve the history of traffic

Soil is not a passive container. Its minerals, organic matter, acidity, moisture and biological activity determine how contaminants behave after deposition.

Metals such as lead, copper and zinc may bind to clay minerals, iron and manganese oxides or organic matter. Depending on soil conditions, they may remain relatively immobile or become more available for transport and biological uptake. Hydrophobic organic compounds can associate with organic matter and fine particles, while some tyre additives and their transformation products can dissolve, degrade or move with water.

Consequently, roadside soil may preserve several overlapping records:

• A deposition record: material delivered by traffic, airborne particles, road runoff and nearby activities.

• A technological record: changes in vehicle components, fuels, brake materials, tyres and road surfaces.

• A spatial record: differences between heavily trafficked roads, residential streets, industrial corridors and relatively undisturbed reference locations.

• A seasonal record: changes associated with rainfall, dry-period accumulation, road cleaning and other environmental processes.

• A management record: the effects of road reconstruction, drainage improvements, soil replacement and contaminated sediment removal.

These records are not equally reliable. Some metals may remain detectable long after their original sources have changed. Organic chemicals may degrade or transform, making their original identity harder to establish. Surface soil may reflect recent deposition, whereas deeper soil may contain older material, but only where the soil profile has remained sufficiently undisturbed.

A chemical archive therefore requires interpretation. The presence of a contaminant establishes that it is there; it does not, by itself, establish when it arrived, where it originated or whether it presents a significant ecological or human-health risk.

How roads transfer pollution into the surrounding environment

Traffic-related contamination develops through several connected processes rather than a single emission pathway.

Tyre and road wear

Tyres lose small quantities of material during normal use. Friction, braking, acceleration, road texture, tyre composition and driving conditions influence this wear. The resulting particles contain rubber polymers, fillers and chemical additives. They may also acquire mineral material and other contaminants after mixing with road dust.

Road surfaces also wear. Asphalt and other pavement materials can contribute particles, while the interaction between tyres and road surfaces creates a mixed material commonly described as tyre-and-road-wear particles (TRWPs).

These particles can settle on road surfaces, become airborne, enter roadside soils or be washed into drainage systems. Their size, shape, density and chemical composition influence how they travel and where they accumulate.

A major challenge is that TRWPs are not always easy to identify using ordinary microscopy or bulk chemical analysis. Their composition varies, and their chemical markers may overlap with those of other sources. Researchers therefore increasingly combine particle characterisation with chemical analysis and source-specific markers.

Brake wear

Braking generates particles from friction between brake components. Depending on the materials used, these particles may contain iron, copper, antimony and other elements. Brake-related particles can settle near roads, contribute to airborne particulate matter and become incorporated into roadside dust.

However, none of these elements is a universally unique marker of brake wear. Copper, for example, may also originate from industrial activity, electrical materials and other urban sources. Reliable source attribution requires multiple lines of evidence.

Resuspended road dust

Road dust contains a mixture of material deposited from traffic, nearby industry, construction, soil erosion, biological sources and atmospheric deposition. Passing vehicles can resuspend this mixture into the air, creating another route of exposure.

The distinction between exhaust and non-exhaust pollution is important here. A particle collected beside a road may have originated from tyre wear, brake wear, pavement degradation, combustion, construction dust or a combination of these processes. Its presence near traffic does not automatically make it a direct vehicle emission.

Runoff and drainage

Rainfall can wash accumulated material from the road surface into adjacent soil, roadside ditches, drains and receiving waters. Depending on the site, contaminants may be transported in dissolved form, attached to suspended particles or carried within larger fragments.

Roadside ditches can therefore act both as sinks and as transport corridors. A contaminant retained in ditch sediment may later be remobilised during intense rainfall, maintenance or erosion. A low concentration in roadside soil does not necessarily mean the surrounding environment has received little pollution; some of the material may have been transported elsewhere.

The chemical fingerprints of traffic

Different traffic-related sources leave different chemical signatures. No single marker is sufficient for every site, but combinations of indicators can help reconstruct likely sources.

Legacy lead: a historical signal with multiple possible origins

Lead is one of the most familiar examples of a persistent environmental contaminant. In many countries, the phase-out of leaded petrol greatly reduced an important source of atmospheric lead emissions. Nevertheless, lead deposited in earlier decades may remain in urban soils and road dust.

Its persistence makes lead potentially useful for historical reconstruction. But roadside lead can also originate from industry, battery-related activities, paint, waste, metal processing and contaminated fill. Modern traffic corridors may sit within industrial landscapes where these sources overlap.

Lead isotopes can sometimes help distinguish sources because different materials may have different isotopic compositions. A recent Delhi study analysed 44 elements in roadside soil and road dust at 33 sites and used lead-isotope ratios alongside rare-earth-element analysis to investigate anthropogenic accumulation and urban pollution patterns. Such methods can strengthen source attribution, but isotopic evidence must be interpreted against plausible local sources and reference materials.

Lead measurements can therefore help identify a historical pollution burden, but they cannot independently date a deposit or prove that all measured lead came from past vehicle emissions.

Platinum-group elements: traces of vehicle technology

Platinum, palladium and rhodium are used in catalytic converters. Small quantities can be released through catalyst wear and related processes, contributing to roadside deposition.

These platinum-group elements are potentially valuable indicators because their use is connected to a particular vehicle technology. Their interpretation is not straightforward, however: concentrations can be low, sources may overlap with industrial emissions, and particle transport can separate the location of deposition from the original emission source.

When combined with traffic history, road distance, soil profiles and other geochemical evidence, these elements may help researchers investigate changes associated with vehicle fleets and pollution-control technology.

Brake wear: copper, iron and antimony

Copper and iron are frequently investigated in studies of brake-related pollution, while antimony has also been used as a potential brake-wear indicator. Their usefulness depends on the materials used in the vehicle fleet and the presence of competing sources.

A strong study should avoid treating one element as conclusive evidence. Instead, it should combine elemental ratios, particle morphology, traffic information, local industrial profiles and appropriate statistical source-apportionment methods.

Tyre wear: zinc, rubber polymers and chemical additives

Zinc has long been associated with tyre materials, but it is not unique to tyres. It may also originate from industrial emissions, galvanised materials and other urban sources. Zinc alone cannot reliably quantify tyre-wear particles.

More specific evidence can come from identifying rubber polymers, characteristic organic compounds or tyre-related transformation products. Even these approaches require care because formulations vary between manufacturers, compounds can change after release, and some markers may have other environmental sources.

Use multiple markers when estimating the contributions of tyre wear, brake wear, and road wear to road dust. Its findings also illustrate why source estimates derived from elemental tracers should not be treated as universal proportions applicable to every city or road.

6PPD-quinone: a pollutant beyond the tyre fragment

One of the most important developments in tyre-pollution research concerns 6PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine) and its transformation product, 6PPD-quinone. (quinone: The oxidation/ozonation transformation product formed when 6PPD reacts with ozone in the atmosphere.)

6PPD is an antioxidant and antiozonant used in tyres to protect rubber from degradation. After release, it can undergo chemical transformation, including oxidation that produces 6PPD-quinone. This means the environmental impact of a tyre cannot be understood solely by counting the particles it sheds: the chemicals associated with those particles may also enter the environment and change over time.

6PPD-quinone can cause acute mortality in coho salmon at environmentally relevant exposures. This discovery helped establish tyre-derived chemicals as an important environmental research area.

The finding must be interpreted within its scope. It does not establish that the same level of toxicity occurs in all fish species, terrestrial organisms or humans. Toxicity varies by organism, exposure conditions and chemical concentration. The extent of long-term risk in many terrestrial environments remains an active research question.

Recent research has moved beyond detecting 6PPD-quinone to examining its transformation and persistence. One study found that 6PPD persisted much longer under anaerobic soil conditions than under aerobic conditions, with reported average half-lives of 51.4 days anaerobically and 0.7–1.5 days aerobically. The researchers also identified transformation products and found that 6PPD-quinone followed different transformation pathways under aerobic and anaerobic conditions.

These results matter for waterlogged roadside soils, submerged sediments and other environments where oxygen availability differs. They show that a tyre-derived chemical's fate cannot be inferred from its behaviour under a single set of laboratory conditions.

Other tyre additives and their transformation products also deserve attention. A study of urban road dust reported additional p-phenylenediamine quinones and found that concentrations of tyre-additive chemicals and tyre-and-road-wear particles varied among road types. A study of road dust and adjacent greenbelt soils also reported rubber-derived chemicals, including diphenylguanidine and benzothiazole-related compounds, with spatial and seasonal variations influenced in part by traffic and rainfall.

The emerging lesson is clear: tyre-related pollution is a family of particles and chemicals, not a single contaminant.

Road surfaces, petroleum residues and mixed urban sources

Roadside soils may also contain petroleum-related hydrocarbons, combustion-derived compounds and material from asphalt and road maintenance. Some of these compounds are associated with traffic; others may arise from heating, industry, fuel handling, construction or nearby waste.

Because roadside environments are mixtures of sources, a credible chemical interpretation should distinguish between evidence of contamination and evidence of its origin.

Why concentration alone can mislead

Two roadside sites can have similar contaminant concentrations yet differ considerably in their environmental significance.

Concentration is the quantity of a contaminant per unit mass of sampled material. It does not reveal the total mass stored across a site, the rate at which new material arrives, or the amount leaving during runoff.

A small patch of soil with a high concentration may contain less total contaminant than a large roadside area with a moderate concentration. Conversely, a low surface concentration can reflect dilution, erosion or transport rather than the absence of an emission source.

The interpretation also depends on particle size. Fine particles may contain elevated concentrations of certain contaminants and can be more readily resuspended or transported. Coarser material may remain near the road or be moved during runoff. Comparing studies that use different size fractions, sampling depths, or analytical methods can therefore lead to misleading conclusions.

Estimates of the relative contributions of tyre, brake and road wear can vary with the indicators and assumptions used. Its reported average estimates based on elemental markers should not be presented as a universal breakdown of road dust.

For reliable comparison, studies should report their sampling strategy, particle-size fraction, analytical detection limits, dry-weight basis, background concentrations and uncertainty. Where possible, they should estimate contaminant loads and transport fluxes in addition to concentrations.

Soil depth: reconstructing a history that may be incomplete

Surface soil often receives the most recent deposition, making it useful for examining current spatial patterns. Deeper layers may contain older contamination, but this interpretation depends on the history of the site.

An undisturbed soil profile may preserve a sequence of deposition and burial. Urban roadside soils, however, are often excavated, levelled, filled, compacted or mixed during road construction and utility work. Earthworms, roots, erosion and water movement can also redistribute material.

A depth profile should therefore be treated as a potential historical record, not an automatic timeline.

Researchers can improve interpretation by combining depth-resolved sampling with soil horizons, sediment characteristics, historical aerial images, road-construction records, traffic data and known changes in fuel or vehicle technology. Where an appropriate dating method is available and justified, it may help constrain the chronology.

Chemical patterns should also be compared with reference sites outside the most direct influence of traffic. Even then, a reference site must be selected carefully: local geology, industrial activity and historical land use can affect background concentrations.

The strongest reconstruction is one in which several independent lines of evidence tell a consistent story.

The monsoon question: where does roadside pollution go in India?

In India, the monsoon makes roadside contamination particularly important because rainfall can rapidly change the balance between accumulation and transport.

During dry periods, road dust and wear particles may build up on road surfaces and adjacent soil. Rain can wash this material away, transfer it into drains, deposit it in roadside depressions or move it into streams and other receiving waters. The outcome depends on rainfall intensity, drainage design, soil permeability, slope, vegetation and the chemical properties of each contaminant.

Some pollutants may be transported mainly with particles. Others may partly dissolve or transform in water. Metals may bind to fine sediments, while some organic chemicals may undergo degradation or sorption. Waterlogged conditions can alter oxygen availability and thereby change the transformation and mobility of particular compounds.

These processes make a simple before-and-after comparison valuable but insufficient. A reduction in roadside-soil concentration after the monsoon could mean that pollution was washed away; it does not prove that the pollutant disappeared from the wider environment. Researchers must investigate where the material went.

A robust Indian field study could sample the same locations before, during and after the monsoon, while also monitoring roadside drains, deposited sediment and nearby receiving waters. Rainfall totals and intensity, traffic volume, road cleaning, drainage maintenance and land use should be recorded. Soil moisture, pH, organic carbon and other relevant properties would help explain changes in chemical behaviour.

Agricultural exposure is another important question. Where roads border fields, transported particles may reach cultivated soil or settle on crops. However, proximity alone does not prove meaningful uptake into edible plant tissue or a health risk. Such claims require measurements of contaminants in soil, plant tissues and, where appropriate, exposure pathways.

India needs more coordinated studies that follow pollutants across connected environmental compartments rather than treating roadside soil as an isolated sample.

What recent Indian studies reveal, and what they do not

Indian research provides an important foundation for this topic, especially because local geology, industrial activity, traffic composition and climate can differ substantially from those in Europe, North America or East Asia.

A 2025 study of road dust in the Durgapur Industrial Area, eastern India, analysed 42 samples for potentially toxic elements and used statistical methods to investigate likely sources. The researchers identified mixed contributions associated with industrial activity and vehicles, while their risk assessment indicated particular concerns for children under the assumptions used (Koley et al., 2025).

A separate study of roadside soil and road dust in Delhi examined 44 elements across 33 sites, including rare-earth elements, and used lead-isotope ratios to investigate anthropogenic accumulation (Siddiqui et al., 2025). A 2026 study in Discover Environment also assessed elemental contamination and potential exposure risks in road dust and roadside soil in residential areas near major industrial areas of Delhi (Siddiqui et al., 2026).

Together, these studies demonstrate why roadside contamination in India should be approached as a mixture of traffic, industrial and broader urban sources. They also show the value of moving beyond total metal measurements to source-apportionment methods and exposure assessment.

Risk estimates require careful explanation. A calculated hazard index or cancer-risk estimate is not a count of observed illnesses. It depends on the measured concentrations, exposure assumptions, toxicity values and model structure. Results from an industrial corridor or a specific Delhi neighbourhood should not be generalised to all Indian cities.

There is also a difference between measuring metals and measuring tyre-derived chemicals. The Indian studies cited above provide useful evidence on elemental contamination, but they do not establish the national distribution of 6PPD-quinone or other tyre additives. Dedicated measurements are needed before conclusions can be drawn about their prevalence and risk in Indian roadside soils.

The research priority is therefore not simply to collect more roadside samples. It is to develop studies that combine reliable source identification, seasonal monitoring, environmental transport and realistic exposure assessment.

From soil contamination to ecological and human-health risk

A contaminant's presence does not automatically establish harm. Risk depends on its concentration, chemical form, bioavailability, exposure route, duration and the sensitivity of the organism or population involved.

For metals, total concentration may not accurately represent the fraction available for biological uptake. Soil chemistry can affect mobility and bioavailability. For organic pollutants, the parent compound may transform into products with different persistence or toxicity.

Potential exposure routes include accidental soil ingestion, inhalation of resuspended dust, contact with contaminated material and, in some settings, transfer into plants or water. Children may have different exposure patterns from adults because of hand-to-mouth behaviour and contact with soil and dust.

These pathways should be assessed with local evidence rather than assumed from proximity to a road. Similarly, the acute toxicity of 6PPD-quinone to a sensitive aquatic species does not establish the magnitude of human-health risk from roadside soil.

A sound assessment separates four questions:

1. Is the contaminant present, and at what concentration?

2. What are its likely sources and environmental pathways?

3. Is it bioavailable, and what exposure is plausible?

4. Does the evidence support a meaningful risk under the conditions being studied?

This approach avoids both extremes: dismissing contamination because harm has not been directly observed, and overstating danger because a chemical has been detected.

Electric vehicles and the changing pollution profile

Electrification can reduce tailpipe emissions, but it does not eliminate pollution from vehicle use.

Tyres still wear, roads still deteriorate and vehicles still generate resuspended dust. Brake-wear emissions can vary with braking systems and driving behaviour; regenerative braking can reduce the use of friction brakes in many electric vehicles, but it does not remove all brake-related emissions.

Vehicle mass, tyre design, acceleration, driving conditions and maintenance can influence wear. The net effect of electrification on non-exhaust pollution therefore cannot be inferred from vehicle type alone.

This matters for the roadside soil archive. As fleets change, exhaust-related signals may decline while tyre-related, brake-related and road-surface particles remain important. Researchers may need to revise their source-identification approaches as vehicle technology evolves.

The policy implication is not that electric vehicles offer no environmental benefit. Rather, transport decarbonisation and non-exhaust pollution control are related but distinct challenges. Cleaner mobility requires attention to both.

The archive is changing: persistence, transformation and redistribution

A roadside soil archive is not fixed. Contaminants may be buried, diluted, eroded, resuspended, leached or transformed. Soil organisms can alter the local environment, while changes in moisture and oxygen can influence chemical reactions.

This is particularly important for tyre-derived chemicals. The 2025 study by Shen et al. showed that 6PPD and 6PPD-quinone can follow different transformation pathways under aerobic and anaerobic conditions. The 2026 research on rubber-derived chemicals in roadside greenbelt soils also found spatial and seasonal patterns consistent with changing inputs and rainfall-related processes.

Research published in October 2026 has further examined seasonal variation in 6PPD and 6PPD-quinone in road dust in a snowy region of Japan. It reported that concentrations varied across the year and were associated with a combination of tyre-use patterns and meteorological conditions, while also acknowledging limitations in the small temporal dataset (Takeuchi & Terasaki, 2026).

The Japanese findings cannot be transferred directly to India’s monsoon climate. They nevertheless illustrate a wider principle: environmental concentrations reflect the balance between new inputs, chemical transformation and removal or redistribution. Seasonal sampling is essential if researchers want to understand that balance.

A chemical detected in soil today may not be the same compound originally released. Conversely, transformation can create products that require their own assessment. Environmental monitoring should therefore consider both parent chemicals and relevant transformation products wherever validated analytical methods are available.

The research frontiers that could make roadside-soil science more powerful

Several questions remain especially important.

First, how accurately can researchers identify sources? Metals often come from multiple sources, and some tyre-derived markers may transform after release. Combining elemental analysis, polymer identification, organic chemical analysis, isotopic evidence and local source profiles may improve attribution, but methods must be validated for the material being studied.

Second, what happens beyond the roadside? A road-edge sample provides only a partial picture. Studies should trace particles and chemicals through soil, drainage networks, sediments and receiving waters. This is particularly important in monsoon environments, where the location of maximum accumulation may change with rainfall and maintenance.

Third, how persistent are emerging tyre chemicals in real soils? Laboratory studies reveal important mechanisms, but natural soils vary in temperature, moisture, microbial activity, organic matter and oxygen availability. Long-term field data are needed to determine how laboratory findings translate to real roadside environments.

Fourth, how do contaminants interact? Roadside soils contain mixtures rather than isolated chemicals. Metals, tyre additives, petroleum-related compounds and other pollutants may coexist. The effects of mixtures cannot always be predicted from the toxicity of individual compounds, so carefully designed mixture studies may be necessary.

Fifth, how does monsoon rainfall affect exposure? A pollutant may decline in surface soil yet increase in drainage sediment or receiving water. Monitoring only one compartment risks confusing redistribution with removal.

Sixth, what is the actual risk to agriculture and people? Research should distinguish total soil concentration from bioavailable fractions, plant uptake, human exposure and demonstrated effects. This is particularly important near residential areas, schools, informal settlements and agricultural land.

Seventh, how will future transport systems change the chemical record? Electric vehicles, new tyre formulations, altered braking systems and changing road materials may change the composition of non-exhaust emissions. Long-term monitoring can help identify these shifts.

These questions offer opportunities for research that is both scientifically novel and practically useful. The strongest contribution will not necessarily be the discovery of a previously unknown chemical. It may be a better method for distinguishing sources, measuring seasonal transport or linking roadside accumulation to downstream exposure.

A practical framework for studying Indian roadside soils

A credible Indian study should begin with a clearly defined question. Is the goal to reconstruct historical deposition, compare road types, quantify tyre-related pollution, investigate monsoon transport or assess potential exposure? The sampling strategy should follow that objective.

A practical design could include:

• Contrasting sites: high-traffic roads, residential roads, industrial corridors and suitable reference locations.

• Distance gradients: samples at the road edge and progressively farther away, with distances selected according to the site.

• Depth profiles: separate surface and subsurface samples where soil has not been heavily disturbed.

• Seasonal sampling: matched pre-monsoon and post-monsoon collections, with additional sampling during or after rainfall where feasible.

• Connected compartments: road dust, roadside soil, drains, ditch sediment and receiving waters where relevant.

• Chemical analysis: a justified suite of metals and, where facilities permit, tyre polymers, 6PPD, 6PPD-quinone and other relevant tyre additives.

• Soil characterisation: pH, organic carbon, texture, moisture and other properties relevant to pollutant mobility.

• Source information: traffic counts, vehicle composition, industrial land use, road age, resurfacing history, drainage conditions and road-cleaning practices.

• Quality assurance: field blanks, laboratory blanks, duplicates, reference materials, detection limits and transparent reporting of uncertainty.

The analysis should distinguish correlation from causation. A relationship between traffic intensity and a metal concentration may be consistent with a traffic source, but industrial activity or local geology could produce a similar pattern. Multivariate statistics can help identify patterns, yet statistical grouping alone does not prove a source.

For emerging organic chemicals, researchers should also consider whether the analytical method can reliably detect the target compounds in complex soil matrices. Non-detection is not proof of absence if the method's detection limit is too high or the compound is not included in the analytical target list.

A study designed this way could produce more than a map of contaminated sites. It could explain how pollutants enter the environment, how they change with season and distance, and where management interventions are most likely to be effective.

From monitoring to management

The chemical archive of roadside soil is useful only if it improves decisions.

Where evidence identifies contaminated dust or soil as a concern, interventions may include improved road and construction-dust management, appropriate stormwater controls, stabilisation of exposed soil, vegetation where suitable, and targeted treatment or removal of contaminated material.

Roadside vegetation can trap some particles and reduce erosion, but its effectiveness depends on species, density, maintenance, soil conditions and pollutant type. Vegetation should not be treated as a universal remedy or used to conceal contaminated soil without assessing the consequences.

Drainage systems also need attention. Sediment traps and suitable treatment systems may reduce the movement of particles into receiving waters, but they require maintenance and safe handling of collected material. Otherwise, contaminants may simply accumulate in a new location and later be released.

Source control remains essential. Better tyre durability, lower-emission materials, improved road design, effective braking systems and appropriate vehicle maintenance may reduce particular non-exhaust emissions. However, proposed interventions should be evaluated for their actual performance, costs and possible unintended consequences.

For India, the most useful policy approach would integrate roadside soil monitoring with road-dust management, stormwater planning, urban soil assessment and catchment-level pollution control. Monitoring should be targeted to locations where exposure or transport is plausible, rather than assuming that every road edge presents the same level of risk.

Roadside soil preserves an incomplete but valuable record of modern transport. Metals may reveal long-lived deposition; tyre and brake particles may indicate ongoing non-exhaust emissions; and emerging tyre-derived chemicals may reveal pollution pathways that conventional monitoring overlooks.

The record must be read cautiously. Contaminants can have multiple sources, soil profiles can be disturbed, chemical signatures can change, and concentrations alone do not establish risk. Yet these limitations do not make roadside soil uninformative. They make careful sampling, source attribution and environmental interpretation essential.

Recent research on tyre-wear particles, 6PPD-quinone and other rubber-derived chemicals has expanded the scientific question from what material is deposited beside roads? to what happens to that material after it enters the environment? The answer involves chemical transformation, seasonal transport, soil properties, drainage networks and interactions among pollutants.

In India, the monsoon provides a particularly important reason to study these connections. A decline in roadside concentration may represent transport rather than elimination; a contaminated road edge may be only one part of a larger environmental pathway. Linking soil with drains, sediments, water and agricultural land will be necessary to understand the full system.

The broader lesson is that the environmental footprint of transport extends beyond exhaust pipes and vehicle-use statistics. Roads continually exchange material with the landscapes they cross. By treating roadside soil as a dynamic chemical archive, and by recognising both what it can reveal and what it cannot prove, environmental science can develop a more complete account of the past, present and future impacts of urban mobility.