Extreme rainfall can make roads difficult to use long before water becomes deep enough to stop traffic. Heavy downpours can reduce visibility, affect vehicle movement, and interfere with traffic signals, creating a more complicated challenge for transportation networks.

A 2026 study published in Reliability Engineering & System Safety examined these combined effects across 26 local government areas in and around Greater London.

Waterlogging Is Only One Part

When intense rainfall is discussed, flooded roads are often the first concern. Water accumulating on a road can reduce vehicle speeds, lower the amount of traffic a road can handle, and in severe cases make sections effectively unusable.

However, the research by Jie Liu, Zizhen Xu, Li Wan, and Kristen MacAskill shows that looking only at waterlogging can miss other important effects. Their analysis considers three factors together: water on roads, reduced visibility, and traffic-signal power failures.

This broader approach matters because a road can remain physically open while still becoming much less effective for moving people. Drivers may slow down because heavy rain makes it harder to see road conditions, other vehicles, signs, or intersections clearly.

Visibility Can Change Road Performance

Reduced visibility is especially important because its effects do not necessarily require flooding. During intense rainfall, drivers generally need more time and space to react to changing conditions. Lower visibility can therefore affect the speed and capacity of road sections even when water levels are relatively limited.

The study found that this factor becomes particularly significant under some less severe rainfall scenarios. For a rainfall event with a 1-in-30-year return period, poor visibility made a substantial contribution to the overall reduction in road performance. This finding suggests that assessments based entirely on flood depth may give an incomplete picture. A road network can experience meaningful disruption even when waterlogging itself is not the dominant problem.

Traffic Signals Add Another Layer

Heavy rainfall can also create problems for infrastructure supporting road operations. The researchers examined the effect of traffic-signal power failures as another possible consequence of extreme rainfall. Unlike reduced visibility, however, signal failures had a comparatively limited influence at the regional network level in the study. Their effects were more localized, particularly in certain urban areas.

That does not mean traffic signals are unimportant. At individual intersections, a signal outage can create delays and make traffic movement more difficult. But when the entire regional network is considered, the study found that this factor contributed less to overall disruption than the other two mechanisms.

What the London Study Found

The researchers tested rainfall scenarios representing 1-in-30-year, 1-in-100-year, and 1-in-1000-year events across a large road network covering Greater London and surrounding areas. One of the clearest findings was the difference between a waterlogging-only assessment and a model that included all three factors. Ignoring visibility and signal failures could overestimate usable road capacity by roughly 15% to 30%, depending on the rainfall scenario.

The effect on overall network efficiency was also important. When only waterlogging was considered, the reduction in network efficiency was underestimated by approximately 1% to 23% across the scenarios examined. The largest difference occurred in the 1-in-30-year scenario, where reduced visibility played a particularly noticeable role.

Why Network Connectivity Matters

Road performance is not simply about whether individual streets remain open. Transportation depends on connections between different areas, so disruption on particular sections can influence journeys across a much wider network.

The study therefore examined both road connectivity and travel efficiency. Waterlogging can sometimes make certain sections unusable, forcing routes to change. At the same time, reduced speeds on otherwise open roads can increase journey times. This distinction helps explain why the impact of extreme rainfall cannot always be estimated by counting flooded road sections alone.

Planning for More Than Flooded Roads

The findings have practical implications for transportation planning. Emergency managers and infrastructure planners need to consider how different rainfall-related problems interact rather than relying exclusively on flood-depth information. A more complete assessment can help identify roads where visibility is likely to become a major concern, locations vulnerable to signal disruptions, and areas where multiple effects could occur at the same time.

Extreme rainfall does not have to completely flood a road to cause serious disruption. By considering what happens on the road as well as the conditions that influence how people travel, planners can develop a more realistic understanding of rainfall-related disruption and make better-informed decisions about future road resilience.