Why one-way streets are not a good solution

 

Why one-way streets are not a good solution

The Served Trips Capacity "STC" is increased when two-way streets are acquired in the street network

 

Based on the established transport planning mentality of the past decades, when it was considered that one-way streets were a solution in order to improve mobility, the trend towards two-way streets in cities one could say that it would have puzzled some people - even experts scientists, traffic engineers!

 

In the USA, the UK and elsewhere, cities such as Dallas, Denver, Sacramento, Tampa, London, Sheffield, etc. have already converted one-way streets into two-way streets in recent years. This shift in transport planning in areas where a network of two-way roads was implemented led to the achievement of:

·         Revitalisation, as traffic flow ιn one-way streets is continuous, making it difficult for cyclists and pedestrians in the area where the one-way streets are located.

·         Facilitation for commuters, as the one-way streets networks on the one hand cause confusion for drivers, which ultimately leads to more vehicle-kilometres travelled, on the other hand, they make it more difficult for bus passengers to locate stops for a return trip

·         Improved safety, as speeds tend to be higher on the one-way streets and some studies show that drivers pay less attention while using them because there is in fact no traffic flow in the opposite direction

·         Economic upgrading of these areas, as local businesses are more visible when driving in two-way streets

 

When the transport planning of cities, in previous decades, was oriented towards one-way streets in order to facilitate traffic flow there was, in fact, some substance to it. Indeed, to a large extent, with one-way streets the difficult left turns, which create a conflict with the opposing traffic, are eliminated. It is also true that any way of avoiding this conflict because of left turns on two-way streets, creates traffic congestion, as the establishment of separate left-turn lanes causes space to be taken up while traffic signal control causes time to be wasted.

 

It was, because now it is no longer the dominant view, as there is a shift in the against one-way approach, as the trend now is for one-way streets to be converted into two-way streets in order for cities to benefit from the above mentioned advantages of areas with two-way streets. In addition to these advantages, two-way streets lead to the improvement of:

·         traffic congestion

·         the Served Trips Capacity "STC"

 

In order to understand these beneficial effects of areas with two-way streets, it should be borne in mind that the traditional traffic flow measurement, with which most people are familiar, offers no more than the information of how many vehicles pass in front of the measurement point. It is reasonable and predictable that the flow is high on one-way streets because there is no reason for vehicles to slow down. However, traffic flow is a quantity that does not take into account the fact that moving through one-way systems often means that a circuitous route through the area must be followed in order to get to the desired destination. This detour adds distance travelled to each trip.

However, by introducing the quantity of the Served Trips Capacity "STC" and measuring it on a street network, one can have a realistic estimate of the serving capacity of the network. The STC is the ratio of two quantities:

·         the traffic flow capacity, which is determined by the number of vehicles that can pass through an intersection during a signal cycle, and

·         the average trip length

 

Well, research shows that two-way street networks can serve more trips per time unit than one-way street networks when the average trip length is short. This is because STC takes into account both traffic flow and additional travelled distance. A comparison of one-way streets with different types of two-way streets (e.g., full left-turn lanes, with left-turn pockets and streets where left turns are banned) is illustrated in the graph below.

 

 

What the graph shows is that, for short trips (origin-destination), two-way streets perform about as good or better than one-way streets, whose traffic flow cannot compensate for the additional trip distance. In the case of longer trip distances, one-way streets start to perform better, but never reach the STC of two-way streets with banned left turns (see the dotted red line). The particularity of this type of system (two-way streets with banned left turns) is that it combines the traffic flow of a one-way street with the directness of a two-way street. Such streets provide the higher speeds that a one-way street would have, but with less trip distance than a one-way street.

 

Conclusion

Cities wishing to improve the "STC" Served Mobility Capacity in some of their areas should:

·         Increase the number of two-way streets regarding areas with short trip lengths

·        Increase the number of two-way streets by banning left turns or at least providing some options for left turns regarding areas with long trip lengths

 

Thus, in any case, one-way streets are not appropriate, and it is advisable to reduce their number in order to increase the commuters serving capacity.