In March 2003, England's Highways Agency began implementation of Managed Motorways (MM), a new traffic management policy to maintain flow through heavily congested sections of M42, a major motorway. MM employs the use of variable mandatory speed limits, dynamic use of the hard shoulder, dedicated Emergency Refuge Areas for vehicles that break down, and gantries with signals and Variable Message Signs (VMS). MM has two main schemes: a Three Lane Variable Mandatory Speed Limit (3L-VMSL), introduced in February 2006, and a Four Lane Variable Mandatory Speed Limit (4L-VMSL), introduced in October 2006. The 3L-VMSL operation is automatic, in which predetermined flow and speed thresholds trigger the variable mandatory speed limit signs to maintain flow. The 4L-VMSL also uses variable mandatory speed limits, but has the optional use of the hard shoulder as a running lane. The hard shoulder is opened for use only by the operators in the control center, when they are confident that the lane is free from obstructions.
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.
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