Published In

Transportation Research Record: Journal of the Transportation Research Board

Document Type


Publication Date



Traffic safety -- Oregon -- Portland, Bicycle traffic flow -- Effect on traffic speed, Bicycle lanes, City traffic -- Oregon -- Portland


A concern raised by some motorists in relation to the presence of bicycles on urban roads without bicycle lanes, discussed in part of the traffic literature, is that cyclists will slow down motorized vehicles and therefore create congestion. This research answers this question: do bicycles reduce passenger car travel speeds on urban roads without bicycle lanes? To answer this question, a detailed comparative analysis of the travel speeds of passenger car (class two vehicles) on lower volume urban roads without bicycle lanes is presented. Speed distributions, the mean, and the 50th and 85th percentile speeds for two scenarios were examined: (i) a passenger car that was preceded by a bicycle and (ii) a passenger car that was preceded by another passenger car. Peak hour traffic and 24-h traffic speeds were analyzed using t-tests and confidence intervals. Although a few statistically significant differences between scenarios (i) and (ii) were found, the actual speed differences were generally in the order of 1 mph or less. Therefore, differences in class two (motorized passenger) vehicle speeds with and without cyclists were found to be negligible from a practical perspective.

Bicycling is a vastly underutilized mode throughout most of the U.S.A., comprising just half of one percent of commuters throughout the nation (1) Given its potential for greater flexibility in route choice and lower costs for infrastructure and operation compared with transit, there is a substantial opportunity for cities to expand bicycling as a primary transportation mode. Congestion mitigation and environmental concerns from rising urban populations have been significant factors cited by communities as they push for greener transportation policies and travel modes.

According to the Portland Bureau of Transportation, in 2017, 6.3% of commuters traveled by bicycle (2). The Portland Bike Plan has established a goal to increase that mode share to 25% by the year 2030 (3). With this mode shift toward bicycling, it is necessary to study the impacts these changes may have on the existing transportation network and motorized vehicles. In support of the Portland Bike Plan’s goal to reach a 25% bicycle mode share, the city authorities expect to add nearly 100 mi (161 km) of bikeways to the existing 385 mi (620 km), approximately 36% of which are currently shared-use roadways (2).

Although it is generally favored to segregate bicyclists and motor vehicles, it is infeasible and often unnecessary to create such infrastructure on every road. For example, Danish bicycle design guidelines suggest that mixed traffic conditions are acceptable for roadways with speed limits less than approximately 35 km/h (22 mph) and average daily traffic (ADT) less than approximately 2,500 vehicles (4).

Shared-use roads can be an economical solution to a growing demand for bicycle facilities. However, this sharing of space presents its own challenges in the contexts of safety and mobility. Several research studies have been conducted on vehicle–bicycle interactions, many of them focused on lateral positioning and passing behavior. Of particular interest, however, is the effect of bicycle traffic on motorized traffic speed, capacity, and flow.

A general concern of motorists in relation to the presence of bicycles on roads without bicycle lanes is that they will impede motor vehicles because of their differing performance characteristics, which may serve to increase congestion and vehicle emissions—two consequences of urbanization that a larger bicycle mode share seeks to mitigate. Recent discussions based on a simulated traffic study have warned that traffic congestion and travel time delay will worsen as the bicycle mode share increases unless bicycle lanes are installed (5, 6). To the authors’ knowledge, there have not been any studies to date using empirical data of passenger cars on shared roads or roads without bicycle lanes that explore the validity of this claim. This paper seeks to expand the knowledge on vehicle–bicycle interactions by studying the impact of bicycles on the travel speed of passenger cars on roadways without bicycle lanes.


This is the Author's Accepted Manuscript of an article that was subsequently published in National Academy of Sciences: Transportation Research Board 2020, June 2020, (1-12), Published by SAGE. The version of record may be found at



Persistent Identifier