Guidelines for road diet conversions

1 Road diets, which convert four-lane highways to three-lane cross sections, are an innovative 2 solution to address mobility and safety concerns under budgetary constraints. These 3 improvements can assist in the development of multimodal corridors with minimal impact on 4 automobile mobility, while retaining the original right of way. Past research has focused on 5 evaluating road diet safety, but minimal guidance exists on determining when such conversions 6 are appropriate from an operational perspective. The proposed guidelines focused on evaluating 7 and comparing the operation of threeand four-lane roads at signalized intersections to provide 8 basic guidance as to when the road diet conversion is appropriate. One of the important findings 9 of this research is the expansion of the usable range for road diets. Prior experience has limited 10 road diet application to roadways with ADTs less than 17,000 vehicles per day. This research 11 identifies the importance of side street volumes and supports the utilization of road diets on 12 roadways with volumes up to 23,000 vehicles per day. This paper provides comprehensive 13 guidance for road diet evaluation including operational performance, correctable safety problems 14 and identifies a list of evaluation elements that should be examined when in-depth analysis of 15 alternatives is required. 16 17 18 19 TRB 2013 Annual Meeting Paper revised from original submittal. INTRODUCTION 1 Rural and urban roadways are becoming increasingly congested throughout the US and other 2 countries, and solutions frequently seek to improve modal options including, bike, pedestrian and 3 transit facilities. A typical approach for solving this problem has been the addition of lanes, but 4 this approach is an expensive and environmentally disruptive practice that frequently offers only 5 short term relief. The need for innovative solutions in addressing mobility and safety concerns in 6 an environment with budgetary constraints is paramount. Such innovative solutions seek to 7 develop multimodal corridors while retaining the original right of way and among them is the 8 concept of road diet, where the number of travel lanes is reduced. Road diets usually involve 9 restriping a four-lane undivided road as a three-lane road with two through lanes and a two-way 10 left-turn lane (TWLTL). This creates surplus roadway width that can be used to to widen existing 11 lanes, create bicycle lanes, supply on street parking, widen sidewalks, or provide opportunities 12 for landscaping and aesthetic improvements. 13 On roadways with high access density, the inside through lane on a four-lane undivided 14 roadway often acts like a de facto turn-lane. This operation can block through traffic, 15 diminishing operations, as well as introduce crash patterns such as rear end crashes and 16 sideswipe crashes resulting from the stopped left turn traffic. The introduction of a TWLTL can 17 often meet the left-turn demand for both directions of travel in a single lane. This modification 18 can then improve safety and mobility by removing turning traffic from the through lanes. 19 Therefore, road diets are a design tool that can be used within existing right of way at a very low 20 cost to improve mobility, and they frequently have no or few negative impacts. 21 Past research has focused primarily on case study evaluation of road diets and on safety 22 performance of these treatments. However, there is little literature providing guidance on the 23 details of the designs or information as to when such conversions will work. The need to identify 24 criteria to be considered for establishing road diets is critical and should be addressed so that 25 state and local agencies can expand their use. A review of all State DOT design manuals did not 26 identify any guidance for road diet conversions, which can hinder their adoption. This study 27 provides such guidance in determining the appropriateness of road diet conversions and 28 identifies parameters to be considered during such evaluations. 29 30 LITERATURE REVIEW 31 Road diets have been shown to improve operating efficiency and safety for all users. The 32 operational efficiency is the result of removing left-turning traffic from through traffic and thus 33 eliminating delays and queues behind turning vehicles while at the same time minimizing lane 34 change maneuvers(due to the single through lane). Case study review has shown road diets to be 35 effective on roads with an average daily traffic (ADT) of up to 25,000 vpd, while other studies 36 have indicated that capacity is not affected by the elimination of the lane and often no increase in 37 congestion is observed (1, 2). Improvements in livability conditions and associated benefits are 38 elements to be considered during road diet conversions, as review of past case studies has 39 indicated (3). Road diets make it easier for pedestrians to cross the road at both signalized and 40 unsignalized intersections, increase feeling of a “safer and more comfortable” street, encourage 41 an increase in pedestrian and bicyclist traffic, and encourage economic growth and 42 redevelopment at a quicker pace (3). 43 A recent study in Iowa evaluated crash data for 30 sites (15 road diets and 15 comparison 44 sites) over a 23 year period (4). The study concluded that, after conversion, there was a 25.2 45 percent reduction in crash frequency and an 18.8 percent reduction in crash rate. Another study 46 using Minnesota road diet conversions also supported safety gains for such roadways (5). Their 47 TRB 2013 Annual Meeting Paper revised from original submittal. study showed reductions of approximately 44 percent in the total number of crashes. In addition, 1 the results showed reductions for injury crashes (45.7 percent) and right angle crashes (37 2 percent). Reductions were also noted for rear end and left-turn crashes but these reductions were 3 not statistically significant. The study also evaluated before-and-after speeds showing reductions 4 both in mean and 85-percentile speeds after the roadway conversion. 5 A study conducted by the FHWA evaluated recent road diet conversions and developed 6 crash modification factors (CMF) for such conversions (6). The study identifies basic benefits of 7 road diets, such as speed reduction, improved safety, and reduced vehicle-pedestrian conflicts. A 8 main goal of the study was to update an earlier study that documented an 18.8 percent reduction 9 in crash rates for road diet conversions (7). The study used a Bayesian approach to compare 10 untreated sites to those where a road diet was implemented. The results clearly indicate that there 11 are safety benefits from these conversions and the magnitude of the benefits depends on the 12 characteristics of the site. Other results include an average reduction of 5 mph in the average 13 speed of motorists and a decrease in percent of drivers driving over 5 mph above the speed limit. 14 The study recommends a CMF of 0.71 for road diet conversions indicating a 29 percent 15 reduction in overall crashes after a road diet installation. 16 The first attempt in defining operational guidelines for road diet conversions was 17 completed in 2001 (8). This effort relied on evaluating before-after conditions on existing road 18 diet projects completed at that time and documented several of the benefits from such 19 conversions. The case studies reviewed showed that the 85-percentile speeds were in general 20 reduced by less than five miles per hour and there was a significant decrease for speeds above 21 the posted speed limit. The review also identified safety improvements with total crash 22 reductions between 17 to 62 percent. The 13 roadway conversions reviewed had ADT volumes 23 of 8,400 to 24,000 vehicles per day (vpd). Simulation was also used to identify potential factors 24 that could be used in determining whether a road diet conversion would be appropriate. The 25 study recommended that a road diet conversion could be considered feasible for roads with an 26 ADT between 15,000 and 17,500 vpd. However, the simulation scenarios only evaluated side 27 street volumes at 40 percent of mainline volumes and three left turn percentages. This limited 28 variation of volume distribution does not provide adequate consideration of potential sites where 29 road diets may be beneficial or even feasible. The authors emphasized that individual detailed 30 analysis is required once such conversions are considered feasible in order to evaluate the 31 corridor conditions and details, such as access frequency and left-turn percentages. The study 32 also identified a list of elements that need to be considered during the evaluation process. 33 A more recent attempt to improve on the prior guidelines for road diets was completed in 34 2006 (9). The main effort was to assess existing road diets and identify the livability benefits 35 from such roadway conversions considering improved mobility for all users and enhanced street 36 character. The resulting effort developed a step by step process in planning, analysis, and 37 implementation of road diet projects and provided a guide for decision-makers. However, the 38 process developed is general in nature and does not provide specific guidance regarding volumes 39 or left-turn percentages indicating when such a project could result in improved operational and 40 safety conditions. 41 Based on this review, the literature is inadequate with respect to providing guidance on 42 implementing road diets. The need to identify criteria to be considered for establishing road 43 diets is critical and should be addressed. 44 45 TRB 2013 Annual Meeting Paper revised from original submittal. METHODOLOGY 1 Typically, road diet conversions will operate at acceptable levels as long as the signalized 2 intersections do not present any operational problems. Therefore, this analysis focused on 3 evaluating and compar