elsewhere, or claiming that bikeways increase urban air pollution. Other transportation planners consider an incremental, piecemeal approach to bike infrastructure buildout ineffective and advocate for complete networks to be built in a single phase. Proponents point out that cycling infrastructure including dedicated bike lanes has been implemented in many cities; when well-designed and well-implemented they are popular and safe, and they are effective at relieving both congestion and air pollution. Bikeway selection Jurisdictions have guidelines around the selection of the right bikeway treatments in order make routes more comfortable and safer for cycling. A study reviewing the safety of "road diets" (motor traffic lane restrictions) for bike lanes found in summary that crash frequencies at road diets in the period after installation were 6% lower, road diets do not affect crash severity, or result in a significant change in crash types. This research was conducted by looking at areas scheduled for conversion before and after the road diet was performed. While also comparing similar areas that had not received any changes. It is noted that further research is recommended to confirm findings. Bikeway types Bikeways can fall into these main categories: separated in-roadway bikeways such as bike lanes and buffered bike lanes; physically separated in-roadway bikeways such as cycle tracks; right-of-way paths such as bike paths and shared use paths; and shared in-roadway bikeways such as bike boulevards, shared lane markings, and advisory bike lanes. The exact categorization changes depending on the jurisdiction and organization, while many just list the types by their commonly used names Dedicated bikeways Sharing with motor traffic Cyclists are legally allowed to travel on many roadways in accordance with the rules of the road for drivers of vehicles. A bicycle boulevard or cycle street is a low speed street which has been optimized for bicycle traffic. Bicycle boulevards discourage cut-through motor vehicle traffic but allow local motor vehicle traffic. They are designed to give priority to cyclists as through-going traffic. A shared lane marking, also known as a sharrow is a street marking that indicates the preferred lateral position for cyclists (to avoid the door zone and other obstacles) where dedicated bike lanes are not available. A 2-1 road is a roadway striping configuration which provides for two-way motor vehicle and bicycle traffic using a central vehicular travel lane and "advisory" bike lanes on either side. The center lane is dedicated to, and shared by,