Two-way left-turn lanes (TWLTLs) and raised medians are the two most common types of median treatments.
A TWLTL is a continuous lane between opposing lanes of traffic to allow traffic to make left turns from both directions, and a raised median is a physical barrier that separates opposing lanes of traffic.
The two median treatments have several advantages and disadvantages in terms of safety and operational performance. TWLTLs remove left-turning vehicles from through lanes, reduce rear-end and sideswipe crashes and provide better access to emergency vehicles. However, TWLTLs often encourage random access, increase serious head-on crashes, do not provide a pedestrian refuge area and could operate poorly on high-volume roads. Compared with TWLTLs, raised medians reduce the number of conflict points at driveways, provide a pedestrian refuge area, reduce head-on crashes and improve mobility of through traffic. However, they limit access to abutting land use, increase left-turn and U-turn traffic at median openings and increase rear-end crashes. Overall, most studies have shown that raised medians are safer than TWLTLs. In spite of the potential safety benefits of raised medians, the business community is known to favor TWLTLs for better accessibility to their businesses.
The Florida Department of Transportation (FDOT) has had a policy to install raised medians in most new multilane highway projects since the 1990s. It is interested in evaluating the safety impacts of median conversion from TWLTLs to raised medians in Florida. FDOT also is interested in the experience of businesses affected by the conversion. Accordingly, the aim of this project is twofold: (1) to quantify the safety impacts of the conversion from TWLTLs to raised medians in Florida; and (2) to find out the experience of businesses on arterials in Florida that were converted from TWLTLs to raised medians.
In this study, the safety impact of the conversion is quantified using a before-and-after analysis of 18 locations (totaling 17.51 miles) that were converted from TWLTLs to raised medians. Police reports of all the crashes up to a maximum of 36 months before and after the median construction were downloaded and reviewed. To document the experience of businesses, a total of 10 locations in commercial areas that were converted from TWLTLs to raised medians were identified. The businesses along these corridors were interviewed.
Overall, the total crash rate across all locations was reduced from 3.6 crashes per million vehicle miles (MVM) to 2.5 crashes per MVM after median conversion, representing a 30.3% reduction in total crash rate. The reductions in crash rate of rear-end, angle, left-turn, right-turn and total crashes were statistically significant; the crash-rate reductions for sideswipe, pedestrian and bicycle crashes were statistically insignificant. There were too few head-on crashes to yield reliable conclusions. In terms of crash severity, there was a statistically significant reduction in property-damage-only (PDO) and injury crash rates. No significant reduction in fatal crash rate was observed after median conversion. Further, reduction in crash rate was observed at both four-lane and six-lane urban arterial facilities and at low-speed and high-speed roadways.
The following is an analysis of raised medians under different roadway and median design features:
Four-lane urban arterials had a mere 4.7% reduction in total crash rate after conversion, while six-lane facilities experienced a 37.2% reduction. From these statistics, it could be inferred that conversion resulted in a greater overall safety benefit for six-lane facilities compared with four-lane facilities. At four-lane facilities, conversion resulted in a reduction in crash rate for all crash types except for rear-end and pedestrian crashes. Specific reasons for the increase in crash rates of these crash types could not be identified. However, review of police reports indicated that a majority of this increase could not be attributed directly to median conversion. Similarly, low-speed and high-speed roadways were analyzed separately; after conversion, total crash rates at low-speed and high-speed roads reduced by 31.8% and 26.5%, respectively.
Type of median opening
Figures 1(a) through 1(d) show the four most common types of median openings. Unidirectional-median opening on a four-lane facility was found to be the safest, and a full-median opening with left-turn bays on both directions on a six-lane facility was found to be the least safe. Among the three types of full-median openings, the bidirectional-median opening with center island type was found to be the safest.
Compared with TWLTLs, raised medians often do not provide enough lateral clearance for errant vehicles. Therefore, one of the safety concerns of constructing raised medians is the frequency of vehicles that directly hit the median curb before stopping or resulting in secondary crashes. Of the 2,436 crashes that occurred at the 18 locations after median construction, only about 2% involved vehicles directly hitting the median curb; 31.3% of these crashes occurred at signalized intersections. The majority of these crashes were not severe. Additionally, of the 2,436 crashes that occurred after median conversion, 1.6% involved vehicles crossing over the median. Further, four-lane facilities were found to have a greater proportion of median crossovers compared with six-lane facilities. At all locations combined, a total of 46 pedestrian crashes occurred after median conversion. None of these 46 pedestrians was hit while standing on the raised median.
Of the 426 businesses that existed at these 10 locations, 151 businesses responded to the interviews that focused on the perception of businesses about raised medians. Figure 2 gives the number of businesses by type that are included in the analysis. Of the 151 businesses, 82 were at mid-block locations without median openings (i.e., with limited access), 42 were at mid-block locations with median openings, and 27 were at signalized intersections.
A majority of the responding businesses preferred TWLTLs to raised medians mainly because TWLTLs provide more access. Several businesses preferred raised medians if they were well designed with a sufficient number of median openings. Several businesses mentioned that there was a decrease in traffic after conversion from TWLTLs to raised medians, and the main reason was accessibility.
Impact of conversion
Only a small percentage of businesses located near signalized intersections believed that medians had an impact on their businesses. This is followed by businesses at mid-block locations with median openings and, finally, businesses at mid-block locations without median openings. Further, as expected, gas stations and auto-service-related businesses mentioned that raised medians had a major impact on their businesses. Also, a high majority of these business types (i.e., gas stations and auto-related businesses) preferred TWLTLs to raised medians.
Among the businesses at mid-block locations without direct access, 41.5% thought that the number of customers decreased after conversion. A relatively high 62.2% indicated that access to their property decreased with median construction, and only 20.7% believed that traffic congestion increased after conversion.
The statistics of businesses at mid-block locations with median opening and near signalized intersections were similar; about one-third of the businesses in each category thought that the number of customers decreased after conversion. A relatively high percentage of businesses believed that traffic congestion increased, mainly because of increased U-turn activity. In terms of access to property, just over half of these businesses believed that access decreased after raised-median construction.
Perception on safety
Of all the responding businesses, 66.7% thought that raised medians were safer than TWLTLs. Among the businesses near signalized intersections, 73.3% perceived raised medians to be safer. Businesses identified better access management and improved pedestrian and vehicle safety as the two main reasons to consider raised medians as being safer than TWLTLs. However, some businesses were under the impression that more crashes had occurred after the construction of raised medians.
Based on the before-and-after safety analysis of 18 locations in this study, it was found that:
Overall, a 30.3% reduction in total crash rate was observed after conversion from TWLTLs to raised medians. The reductions in crash rate of rear-end, angle, left-turn, right-turn and total crashes were statistically significant. In terms of crash severity, there was a statistically significant reduction in PDO and injury crash rates;
A unidirectional-median opening on a four-lane facility was found to be the safest and a full-median opening with left-turn bays in both directions on a six-lane facility was found to be the least safe;
After conversion from TWLTLs to raised medians, four-lane facilities experienced a 4.7% reduction in total crash rate compared with 37.2% reduction on six-lane arterials;
Of the 2,436 crashes that occurred at the 18 study locations after median conversion through December 2010, only 2% (48 crashes) were caused by vehicles hitting the median curb. Further, 38 of these 48 crashes crossed over the median; and
The before-and-after pedestrian-crash statistics showed a 28.9% reduction in pedestrian-crash rate after median conversion, from 63 crashes in the before period to 46 crashes in the after period. Of these 46 crashes, none of the pedestrians was hit while standing on the raised median.
From the on-site interview responses from 151 businesses located along the 10 corridors that were converted from TWLTLs to raised medians, it was found that:
Of the 63 responding businesses, 66.7% thought that raised medians were safer than TWLTLs. Among the businesses located near signalized intersections, 73.3% perceived raised medians to be safer. Businesses identified better access management and improved pedestrian and vehicle safety as the two main reasons to consider raised medians as being safer than TWLTLs;
The majority of businesses (68%) that preferred TWLTLs cited accessibility as the main reason for their preference. Likewise, the majority of businesses (61.5%) that preferred raised medians cited safety as the main reason for their preference; and
Only a small percentage of businesses located near signalized intersections believed that medians had an impact on their businesses. Further, as expected, gas stations and auto-service-related businesses mentioned that raised medians had a major impact on their businesses.
The full report can be accessed at www.dot.state.fl.us/research-center/Completed_Proj/Summary_PL/FDOT-BDK80-977-18-rpt.pdf. ST