Implementing variable mandatory speed limits on four lanes with the optional use of the hard shoulder as a running lane resulted in a 55.7 percent decrease in the number of personal injury accidents on a major motorway in England.
Safety benefits realized from a managed lane approach to congestion relief on M42, England.
A safety analysis of the implementation of a four-lane, variable mandatory speed limit operation (4L-VMSL) and optional use of the hard shoulder as a running lane used a before/after approach. The analysis used data from crashes that occurred on M42 in either direction over an eleven year period, including five years before and three years after the implementation of 4L-VMSL. The analysis reviewed 36 months of data for the 4L-VMSL period and six months of data for the 3L-VMSL period.
The analysis found that the variable mandatory speed limit strategy had a positive impact on safety, resulting in a 55.7 percent reduction in the number of Personal Injury Accidents (PIA) during the first 36 months of 4L-VMSL compared to 3L-VMSL and no variable speed limit (NO-VSL) operations. The first 36 months of 4L-VMSL recorded 81 PIAs, which was less than the number recorded in 3L-VMSL (prorated to 114) and NO-VSL (prorated to 183). On a monthly basis, 4L-VMSL had an average of 2.25 PIAs, compared to 3.17 and 5.08 per month in 3L-VMSL and NO-VSL, respectively.
An analysis of the severity of crashes also found a positive safety impact in 4L-VMSL operations. 4L-VMSL crashes tended to be less severe, and with fewer fatalities, resulting in a drop in the fatality rate from 0.82 per month in 3L-VMSL and NO-VSL, to 0.17 per month in 4L-VMSL. The average index for crash severity was also lower in 4L-VMSL at 0.07, compared to the NO-VSL and 3L-VMSL average index of 0.16. Likewise, the casualty severity index improved, from an average of 0.14 in NO-VSL and 0.11 in 3L-VMSL, to 0.05 in 4L-VMSL.
Although 4L-VMSL had a safety benefit for severe crashes and fatalities, it did not show a reduction in all crash types. While the number of "rear shunt" (rear end) crashes remained fairly constant across all operation types, the proportion of side impact type collisions increased, from 16.1 percent for NO-VSL to 30.9 percent for 4L-VMSL. Thus, there may be a trade-off in terms of the type of crashes that occur as a result of traffic management strategies.
Author: Arlow, A. andMacDonald, M.
Published By: Highways Agency, Department for Transport, UK
Source Date: January 2011URL: http://www.highways.gov.uk/knowledge_compendium/assets/documents/Portfolio/HCG-HRG_264763_001b_V2__2_.doc.pdf
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VSL, managed lanes, congestion mitigation, shoulder running, variable mandatory speed limit